docs(sdk-metrics): fix badge and reword introductory sentence

[pichlermarc]

Which problem is this PR solving?

The badge still showed the version of the outdated "@opentelemetry/metrics" package. The introductory sentence mentioned other Metrics SDKs (node + browser) that were originally planned when this package was still named @opentelemetry/sdk-metrics-base, but tuned out not to be necessary.

This PR fixes both these issues. :slightly_smiling_face:

[codecov[bot]]

Codecov Report

Merging #3484 (c363460) into main (126ae93) will decrease coverage by 0.01%. The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3484      +/-   ##
==========================================
- Coverage   93.79%   93.78%   -0.02%     
==========================================
  Files         249      249              
  Lines        7621     7621              
  Branches     1588     1588              
==========================================
- Hits         7148     7147       -1     
- Misses        473      474       +1     

Impacted Files	Coverage Δ
...-trace-base/src/platform/node/RandomIdGenerator.ts	87.50% <0.00%> (-6.25%)	:arrow_down: