Stabilize synchronous gauge (#4019)

Author: jack-berg

CHANGELOG.md

@@ -19,6 +19,8 @@ release.
   ([#3837](https://github.com/open-telemetry/opentelemetry-specification/pull/3837))
 - Mark Exemplars as stable.
   ([#3870](https://github.com/open-telemetry/opentelemetry-specification/pull/3870))
+- Mark synchronous gauge as stable.
+  ([#4019](https://github.com/open-telemetry/opentelemetry-specification/pull/4019))
 
 ### Logs
 

specification/metrics/api.md

@@ -805,8 +805,6 @@ httpServerDuration.Record(100, new HttpRequestAttributes { method = "GET", schem
 
 ### Gauge
 
-**Status**: [Experimental](../document-status.md)
-
 `Gauge` is a [synchronous Instrument](#synchronous-instrument-api) which can be
 used to record non-additive value(s) (e.g. the background noise level - it makes
 no sense to record the background noise level value from multiple rooms and sum