Introduce "split" metric schema transformation

This is a new transformation type that allows to describe a change
where a metric is converted to several other metrics by eliminating
an attribute.

An example of such change that happened recently is this:
#2617

This PR contains:
- A new "split" transformation added to the schema file format.
- Schema file format incremented from 1.0.0 to 1.1.0. This is considered
  a backward compatible change and thus the minor version number is
  incremented.
- Deleted incorrect sentence that claimed that the addition of a new
  transformation type is not backward compatible and requires a
  major version number change. This is incorrect. Addition of
  new transformation types is backward compatible according to our
  current definition of backward compatibility, which is:
  "consumers can consume version X.Y and older versions X.Z
  provided that they are aware of version X.Y".
- Deleted the requirement that a full OTEP is always necessary to
  introduce new transformation types. This seems excessive. I think
  in simple cases like this a PR directly in the spec is sufficient.

This draft PR shows how the corresponding implementation of schema file
parser in Go can be done:
open-telemetry/opentelemetry-go#2999

This commit in my personal repo shows in more detail how the "split"
transformation can be implemented:
tigrannajaryan/telemetry-schema@2b45e48

CHANGELOG.md

@@ -56,6 +56,9 @@ release.
 
 ### Telemetry Schemas
 
+- Introduce "split" metric schema transformation
+  ([#2653](https://github.com/open-telemetry/opentelemetry-specification/pull/2653)).
+
 ### Common
 
 - Introduce Instrumentation Scope Attributes

specification/schemas/file_format_v1.0.0.md → specification/schemas/file_format_v1.1.0.md

@@ -12,8 +12,8 @@ this schema version.
 Here is the structure of the Schema File:
 
 ```yaml
-# Defines the file format. MUST be set to 1.0.0.
-file_format: 1.0.0
+# Defines the file format. MUST be set to 1.1.0.
+file_format: 1.1.0
 
 # The Schema URL that this file is published at. The version number in the URL
 # MUST match the highest version number in the "versions" section below.
@@ -256,6 +256,27 @@ sections. Here is the structure:
               # specified below.
 ```
 
+#### split Transformation
+
+This transformation splits a metric into several metrics and eliminates an attribute.
+Here is the structure:
+
+```yaml
+    metrics:
+      changes:
+        - split:
+            # Name of old metric to split.
+            apply_to_metric: 
+            # Name of attribute in the old metric to use for splitting. The attribute will be
+            # eliminated, the new metric will not have it.
+            by_attribute: 
+            # Names of new metrics to create, one for each possible value of attribute.
+            attributes_to_metrics:
+              # map of key/values. The keys are the old attribute value used
+              # in the previous version, the values are the new metric name
+              # starting from this version.
+```
+
 ### logs Section
 
 "logs" section in the schema file defines transformations that are applicable
@@ -324,8 +345,8 @@ file. The format version follows the MAJOR.MINOR.PATCH format, similar to semver
 The "file_format" setting is used by consumers of the file to know if they are
 capable of interpreting the content of the file.
 
-The current value for this setting is "1.0.0". Any change to this number MUST
-follow the OTEP process and be published in the specification.
+The current value for this setting is "1.1.0". Any change to this number MUST
+be published in the specification.
 
 The current schema file format allows representing a limited set of
 transformations of telemetry data. We anticipate that in the future more types
@@ -353,19 +374,22 @@ change according to the following rules:
   in a backward compatible manner. "Backward compatible" in this context means
   that consumers that are aware of the new MINOR number can consume the file of
   a particular MINOR version number or of any MINOR version number lower than
-  that, provided that MAJOR version numbers are the same. Typically, this means
-  that the added setting in file format is optional and the default value of the
-  setting matches the behavior of the previous file format version.
+  that, provided that MAJOR version numbers are the same. This can happen for
+  example when:
+
+  - A new transformation type is added.
+
+  - A new setting is added to an existing transformation. The new setting is optional
+    and the default value of the setting matches the behavior of the previous file
+    format version.
 
   Note: there is no "forward compatibility" based on MINOR version number.
   Consumers which support reading up to a particular MINOR version number SHOULD
   NOT attempt to consume files with higher MINOR version numbers.
 
 - MAJOR number SHOULD be increased if the file format is changed in an
-  incompatible way. For example adding a new transformation type in the
-  "changes" section is an incompatible change because it cannot be ignored by
-  existing schema conversion logic, so such a change will require a new MAJOR
-  number.
+  incompatible way. This means the consumers of the file need to parse or interpret
+  the file differently compared to previous MAJOR version.
 
 Correspondingly:
 
@@ -426,8 +450,8 @@ To illustrate this with some examples:
 ## Appendix A. Example Schema File
 
 ```yaml
-# Defines the file format. MUST be set to 1.0.0.
-file_format: 1.0.0
+# Defines the file format. MUST be set to 1.1.0.
+file_format: 1.1.0
 
 # The Schema URL that this file is published at. The version number in the URL
 # MUST match the highest version number in the "versions" section below.
@@ -527,6 +551,20 @@ versions:
               - system.memory.utilization
               - system.paging.usage
 
+        - split:
+            # Example from the change done by https://github.com/open-telemetry/opentelemetry-specification/pull/2617
+            # Name of old metric to split.
+            apply_to_metric: system.paging.operations
+            # Name of attribute in the old metric to use for splitting. The attribute will be
+            # eliminated, the new metric will not have it.
+            by_attribute: direction
+            # Names of new metrics to create, one for each possible value of attribute.
+            attributes_to_metrics:
+              # If "direction" attribute equals "in" create a new metric called "system.paging.operations.in".
+              in: system.paging.operations.in
+              # If "direction" attribute equals "out" create a new metric called "system.paging.operations.out".
+              out: system.paging.operations.out
+
     logs:
       # Definitions that apply to LogRecord data type.
       changes:

specification/schemas/overview.md

@@ -74,7 +74,7 @@ work together.
 Telemetry Schemas are central to how we make this possible. Here is a summary of
 how the schemas work:
 
-- OpenTelemetry defines a [file format](file_format_v1.0.0.md) for defining
+- OpenTelemetry defines a [file format](file_format_v1.1.0.md) for defining
   telemetry schemas.
 
 - Telemetry schemas are versioned. Over time the schema may evolve and telemetry
@@ -182,7 +182,7 @@ passes through the Collector is converted to that target schema:
 ## Schema URL
 
 Schema URL is an identifier of a Schema. The URL specifies a location of a
-[Schema File](file_format_v1.0.0.md) that can be retrieved (so it is a URL and
+[Schema File](file_format_v1.1.0.md) that can be retrieved (so it is a URL and
 not just a URI) using HTTP or HTTPS protocol.
 
 Fetching the specified URL may return an HTTP redirect status code. The fetcher
@@ -215,7 +215,7 @@ Version number follows the MAJOR.MINOR.PATCH format, similar to semver 2.0.
 
 Version numbers use the [ordering rules](https://semver.org/#spec-item-11)
 defined by semver 2.0 specification. See how ordering is used in the
-[Order of Transformations](file_format_v1.0.0.md#order-of-transformations). Other than the ordering
+[Order of Transformations](file_format_v1.1.0.md#order-of-transformations). Other than the ordering
 rules the schema version numbers do not carry any other semantic meaning.
 
 OpenTelemetry schema version numbers match OpenTelemetry specification version

specification/versioning-and-stability.md

@@ -170,7 +170,7 @@ currently described and are allowed:
 - Renaming of span events.
 
 All such changes MUST be described in the OpenTelemetry
-[Schema File Format](schemas/file_format_v1.0.0.md) and published in this repository.
+[Schema File Format](schemas/file_format_v1.1.0.md) and published in this repository.
 For details see [how OpenTelemetry Schemas are published](schemas/overview.md#opentelemetry-schema).
 
 See the [Telemetry Stability](telemetry-stability.md) document for details on how