docs: add Metrics documentation to project (#3360)

Co-authored-by: Chengzhong Wu <legendecas@gmail.com>
Co-authored-by: Marc Pichler <marcpi@edu.aau.at>
Co-authored-by: Marc Pichler <marc.pichler@dynatrace.com>
Co-authored-by: Daniel Dyla <dyladan@users.noreply.github.com>

CHANGELOG.md

@@ -44,6 +44,7 @@ For experimental package changes, see the [experimental CHANGELOG](experimental/
 
 ### :books: (Refine Doc)
 
+* doc: Added Metrics documentation [#3360](https://github.com/open-telemetry/opentelemetry-js/pull/3360) @weyert
 * docs(api): fix counter negative value wording [#3396](https://github.com/open-telemetry/opentelemetry-js/pull/3396) @legendecas
 
 ### :house: (Internal)