Mark relevant portions of Metrics DataModel stable (#1728)

* Mark sections of datamodel stable.

* Generated ToC

* Clean up MUST and SHOULD in the document.

* adjust for bug in markdown-toc

* Mark protocol as stable.

Co-authored-by: Carlos Alberto Cortez <calberto.cortez@gmail.com>

specification/protocol/otlp.md

@@ -51,7 +51,7 @@ Each signal has different support and stability in OTLP, described through its
 own maturity level, which in turn applies to **all** the OTLP Transports listed below.
 
 * Tracing: **Stable**
-* Metrics: **Beta**
+* Metrics: **Stable**
 * Logs: **Alpha**
 
 See [OTLP Maturity Level](https://github.com/open-telemetry/opentelemetry-proto#maturity-level).