Export SpanContext.IsRemote in OTLP (open-telemetry#182)

* Introduce remote-parent OTEP

oteps/0182-otlp-remote-parent.md

@@ -0,0 +1,124 @@
+# Export SpanContext.IsRemote in OTLP
+
+Update OTLP to indicate whether a span's parent is remote.
+
+## Motivation
+
+It is sometimes useful to post-process or visualise only entry-point spans: spans which either have no parent (trace roots), or which have a remote parent.
+For example, the Elastic APM solution highlights entry-point spans (Elastic APM refers to these as "transactions") and surfaces these as top-level operations
+in its user interface.
+
+The goal is to identify the spans which represent a request that is entering a service, or originating within a service, without having to first assemble the
+complete distributed trace as a DAG (Directed Acyclic Graph). It is trivially possible to identify trace roots, but it is not possible to identify spans with
+remote parents.
+
+Here is a contrived example distributed trace, with a border added to the entry-point spans:
+
+```mermaid
+graph TD
+    subgraph comments_service
+    POST_comments(POST /comment)
+    POST_comments --> comments_send(comments send)
+    end
+
+    subgraph auth_service
+    POST_comments --> POST_auth(POST /auth)
+    POST_auth --> LDAP
+    end
+
+    subgraph user_details_service
+    POST_comments --> GET_user_details(GET /user_details)
+    GET_user_details --> SELECT_users(SELECT FROM users)
+    end
+
+    subgraph comments_inserter
+    comments_send --> comments_receive(comments receive)
+    comments_receive --> comments_process(comments process)
+    comments_process --> INSERT_comments(INSERT INTO comments)
+    end
+
+    style POST_comments stroke-width:4
+    style POST_auth stroke-width:4
+    style GET_user_details stroke-width:4
+    style comments_receive stroke-width:4
+```
+
+## Explanation
+
+The OTLP encoding for spans has a boolean `parent_span_is_remote` field for identifying whether a span's parent is remote or not.
+All OpenTelemetry SDKs populate this field, and backends may use it to identify a span as being an entry-point span.
+A span can be considered an entry-point span if it has no parent (`parent_span_id` is empty), or if `parent_span_is_remote` is true.
+
+## Internal details
+
+The first part would be to update the trace protobuf, adding a `boolean parent_span_is_remote` field to the
+[`Span` message](https://github.com/open-telemetry/opentelemetry-proto/blob/b43e9b18b76abf3ee040164b55b9c355217151f3/opentelemetry/proto/trace/v1/trace.proto#L84).
+
+[`SpanContext.IsRemote`](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/api.md#isremote) identifies whether span context has been propagated from a remote parent.
+The OTLP exporter in each SDK would need to be updated to record this in the new `parent_span_is_remote` field.
+
+For backwards compatibility with older OTLP versions, the protobuf field should be `nullable` (`true`, `false`, or unspecified)
+and the opentelemetry-collector protogen code should provide an API that enables backend exporters to identify whether the field is set.
+
+```go
+package pdata
+
+// ParentSpanIsRemote indicates whether ms's parent span is remote, if known.
+// If the parent span remoteness property is known then the "ok" result will be true,
+// and false otherwise.
+func (ms Span) ParentSpanIsRemote() (remote bool, ok bool)
+```
+
+## Trade-offs and mitigations
+
+None identified.
+
+## Prior art and alternatives
+
+### Alternative 1: include entry-point span ID in other spans
+
+As an alternative to identifying whether the parent span is remote, we could instead encode and propagate the ID of the entry-point span in all non entry-point spans.
+Thus we can identify entry-point spans by lack of this field.
+
+The entry-point span ID would be captured when starting a span with a remote parent, and propagated through `SpanContext`. We would introduce a new `entry_span_id` field to
+the `Span` protobuf message definition, and set it in OTLP exporters.
+
+This was originally [proposed in OpenCensus](https://github.com/census-instrumentation/opencensus-specs/issues/229) with no resolution.
+
+The drawbacks of this alternative are:
+
+- `SpanContext` would need to be extended to include the entry-point span ID; SDKs would need to be updated to capture and propagate it
+- The additional protobuf field would be an additional 8 bytes, vs 1 byte for the boolean field
+
+The main benefit of this approach is that it additionally enables backends to group spans by their process subgraph.
+
+### Alternative 2: introduce a semantic convention attribute to identify entry-point spans
+
+As an alternative to adding a new field to spans, a new semantic convention attribute could be added to only entry-point spans.
+
+This approach would avoid increasing the memory footprint of all spans, but would have a greater memory footprint for entry-point spans.
+The benefit of this approach would therefore depend on the ratio of entry-point to internal spans, and may even be more expensive.
+
+### Alternative 3: extend SpanKind values
+
+Another alternative is to extend the SpanKind values to unambiguously define when a CONSUMER span has a remote parent or a local parent (e.g. with the message polling use case).
+
+For example, introducing a new SpanKind (e.g. `AMBIENT_CONSUMER`) that would have a clear `no` on the `Remote-Incoming` property of the SpanKind, and `REMOTE_CONSUMER` would have a clear `yes` on the `Remote-Incoming` property of the SpanKind. The downside of this approach is that it is a breaking on the semantics of `CONSUMER` spans.
+
+## Open questions
+
+### Relation between `parent_span_is_remote` and `SpanKind`
+
+The specification for `SpanKind` describes the following:
+
+```
+The first property described by SpanKind reflects whether the Span is a "logical" remote child or parent ...
+```
+
+However, the specification stay ambiguous for the `CONSUMER` span kind with respect to the property of the "logical" remote parent.
+Nevertheless, the proposed field `parent_span_is_remote` has some overlap with that `SpanKind` property.
+The specification would require some clearification on the `SpanKind` and its relation to `parent_span_is_remote`.
+
+## Future possibilities
+
+No other future changes identified.