Add Context parameter to Enabled for synchronous metrics instruments

Fixes open-telemetry#4256

specification/metrics/api.md

@@ -483,9 +483,12 @@ To help users avoid performing computationally expensive operations when
 recording measurements, [synchronous Instruments](#synchronous-instrument-api)
 SHOULD provide this `Enabled` API.
 
-There are currently no required parameters for this API. Parameters can be
-added in the future, therefore, the API MUST be structured in a way for
-parameters to be added.
+The API SHOULD accept the following parameters:
+
+- The [Context](../context/README.md) to be associated with the measurement.
+  When implicit Context is supported, then this parameter SHOULD be optional and
+  if unspecified then MUST use current Context.
+  When only explicit Context is supported, accepting this parameter is REQUIRED.
 
 This API MUST return a language idiomatic boolean type. A returned value of
 `true` means the instrument is enabled for the provided arguments, and a returned