Clarify a default-enabled behavior for Metric Views

CHANGELOG.md

@@ -33,6 +33,10 @@ release.
   ([#2379](https://github.com/open-telemetry/opentelemetry-specification/pull/2379)).
 - Clarify that MetricReader has one-to-one mapping to MeterProvider.
   ([#2406](https://github.com/open-telemetry/opentelemetry-specification/pull/2406)).
+- Clarify that instruments are enabled by default, even when Views are configured.
+  Require support for the match-all View expression having `name=*` to support 
+  disabling instruments by default.
+  ([#2417](https://github.com/open-telemetry/opentelemetry-specification/pull/2417)).
 
 ### Logs
 

spec-compliance-matrix.md

@@ -165,6 +165,8 @@ Disclaimer: this list of features is still a work in progress, please refer to t
 | The updated configuration applies to all already returned `Meter`s.                                                                                                          | if above | -  |  -   |    |    -   |      |        |     |      |     |   +  |       |
 | There is a way to register `View`s with a `MeterProvider`.                                                                                                                   |          | -  |  +   |    |    +   |      |        |     |      |     |   +  |       |
 | The `View` instrument selection criteria is as specified.                                                                                                                    |          |    |  +   |    |    -   |      |        |     |      |     |   +  |       |
+| The `View` instrument selection criteria supports wildcards.                                                                                                                 | X        |    |      |    |        |      |        |     |      |     |      |       |
+| The `View` instrument selection criteria supports the match-all wildcard.                                                                                                    |          |    |      |    |        |      |        |     |      |     |      |       |
 | The name of the `View` can be specified.                                                                                                                                     |          |    |  +   |    |    -   |      |        |     |      |     |   +  |       |
 | The `View` allows configuring the name description, attributes keys and aggregation of the resulting metric stream.                                                          |          |    |  ?   |    |    -   |      |        |     |      |     |   -  |       |
 | The `View` allows configuring the exemplar reservoir of resulting metric stream.                                                                                             | X        |    |  ?   |    |    -   |      |        |     |      |     |   -  |       |

specification/metrics/sdk.md

@@ -151,7 +151,9 @@ are the inputs:
   * The `name` of the Instrument(s). [OpenTelemetry SDK](../overview.md#sdk)
     authors MAY choose to support wildcard characters, with the question mark
     (`?`) matching exactly one character and the asterisk character (`*`)
-    matching zero or more characters.
+    matching zero or more characters.  If wildcards are not supported in general, 
+	OpenTelemetry SDKs MUST specifically recognize the single `*` wildcard 
+	as matching all instruments.
   * The `name` of the Meter (optional).
   * The `version` of the Meter (optional).
   * The `schema_url` of the Meter (optional).
@@ -210,11 +212,9 @@ made with an Instrument:
       know (e.g. expose
       [self-diagnostics logs](../error-handling.md#self-diagnostics)).
   * If the Instrument could not match with any of the registered `View`(s), the
-    SDK SHOULD provide a default behavior. The SDK SHOULD also provide a way for
-    the user to turn off the default behavior via MeterProvider (which means the
-    Instrument will be ignored when there is no match). Individual
-    implementations can decide what the default behavior is, and how to turn the
-    default behavior off.
+    SDK SHOULD enable the instrument using the default aggregation and temporality.
+	Users can configure match-all Views using Drop aggregation to disable instruments 
+	by default.
 * END.
 
 Here are some examples: