Clarify the metrics spec status (#1401)

* clarify the metrics spec status

* update changelog

Co-authored-by: Bogdan Drutu <bogdandrutu@gmail.com>

CHANGELOG.md

@@ -14,6 +14,9 @@ New:
 
 Updates:
 
+- Remove Metrics SDK specification to avoid confusion, clarify that Metrics API
+  specification is not recommended for client implementation
+  ([#1401](https://github.com/open-telemetry/opentelemetry-specification/pull/1401))
 - Rename OTEL_TRACE_SAMPLER and OTEL_TRACE_SAMPLER_ARG env variables to OTEL_TRACES_SAMPLER and OTEL_TRACES_SAMPLER_ARG
   ([#1382](https://github.com/open-telemetry/opentelemetry-specification/pull/1382))
 - Mark some entries in compliance matrix as optional([#1359](https://github.com/open-telemetry/opentelemetry-specification/pull/1359))

README.md

@@ -36,7 +36,6 @@ Technical committee holds regular meetings, notes are held
   - [Metrics](specification/metrics/api.md)
 - SDK Specification
   - [Tracing](specification/trace/sdk.md)
-  - [Metrics](specification/metrics/sdk.md)
   - [Resource](specification/resource/sdk.md)
   - [Configuration](specification/sdk-configuration.md)
 - Data Specification

specification/metrics/api.md

@@ -2,6 +2,11 @@
 
 **Status**: [Experimental](../document-status.md)
 
+Note: this specification is subject to major changes. To avoid thrusting
+language client maintainers, we don't recommend OpenTelemetry clients to start
+the implementation unless explicitly communicated via
+[OTEP](https://github.com/open-telemetry/oteps#opentelemetry-enhancement-proposal-otep).
+
 <!-- toc -->
 
 - [Overview](#overview)