Stabilize Enabled API for synchronous instruments (#4746)

Fixes
#4215

Current implementation status:

| Language | Status | Where it is / code reference |
| ----------------- | ---------------------- |
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|
| **Go** | **Yes (experimental)** | Implemented on synchronous
instruments in the SDK’s experimental `x` package: “Instrument
**Enabled** synchronous instruments provide an **Enabled** method.” See
the Go package docs. ([1]) |
| **.NET** | **Yes (in BCL)** | Use
**`System.Diagnostics.Metrics.Instrument.Enabled`**: “Gets a value that
indicates if there are any listeners for this instrument.” This is the
.NET runtime Metrics API that OpenTelemetry .NET uses. ([2]) |
| **Java** | **Yes (incubator)** | Implemented on synchronous
instruments in the incubator API. ([3]) |
| **JavaScript** | **No** | |
| **Python** | **No** | |
| **C++** | **No** | |
| **Rust** | **No** | |
| **Ruby** | **No** | |
| **PHP** | **No** | |
| **Swift** | **No** | |
| **Erlang/Elixir** | **No** | |

[1]: https://pkg.go.dev/go.opentelemetry.io/otel/sdk/metric/internal/x
"go.opentelemetry.io/otel/sdk/metric/internal/x - Go Packages"
[2]:
https://learn.microsoft.com/en-us/dotnet/api/system.diagnostics.metrics.instrument.enabled?view=net-9.0
"Instrument.Enabled Property (System.Diagnostics.Metrics)"
[3]:
https://github.com/open-telemetry/opentelemetry-java/tree/main/api/incubator/src/main/java/io/opentelemetry/api/incubator/metrics

Note that @open-telemetry/collector-maintainers would benefit from
making it stable and would not have to do things like
https://github.com/open-telemetry/opentelemetry-collector/blob/2e9c82787618ea7da3fada176abb45b5a30714bf/service/internal/obsconsumer/enabled.go.

---------

Co-authored-by: Carlos Alberto Cortez <[email protected]>

Author: pellared

CHANGELOG.md

@@ -18,6 +18,8 @@ release.
 
 ### Metrics
 
+- Stabilize `Enabled` API for synchronous instruments.
+  ([#4746](https://github.com/open-telemetry/opentelemetry-specification/pull/4746))
 - Allow instrument `Enabled` implementation to have additional optimizations and features.
   ([#4747](https://github.com/open-telemetry/opentelemetry-specification/pull/4747))
 

specification/metrics/api.md

@@ -472,16 +472,14 @@ or something else).
 
 ### General operations
 
-All instruments SHOULD provide functions to:
+All [synchronous instruments](#synchronous-instrument-api) SHOULD provide functions to:
 
 * [Report if instrument is `Enabled`](#enabled)
 
 #### Enabled
 
-**Status**: [Development](../document-status.md)
-
 To help users avoid performing computationally expensive operations when
-recording measurements, [synchronous Instruments](#synchronous-instrument-api)
+recording measurements, [synchronous instruments](#synchronous-instrument-api)
 SHOULD provide this `Enabled` API.
 
 There are currently no required parameters for this API. Parameters can be