Declare Trace part of OTLP Stable (#160)

Resolves: #154

README.md

@@ -11,11 +11,11 @@ See [contribution guidelines](CONTRIBUTING.md) if you would like to make any cha
 Component                | Maturity |
 -------------------------|----------|
 collector/metrics/*      | Alpha    |
-collector/trace/*        | Beta     |
+collector/trace/*        | Stable   |
 common/*                 | Beta     |
 metrics/*                | Alpha    |
 resource/*               | Beta     |
-trace/trace.proto        | Beta     |
+trace/trace.proto        | Stable   |
 trace/trace_config.proto | Alpha    |
 
 (See [maturity-matrix.yaml](https://github.com/open-telemetry/community/blob/47813530864b9fe5a5146f466a58bd2bb94edc72/maturity-matrix.yaml#L57)