feat(api-metrics): remove observable types

[legendecas]

Which problem is this PR solving?

Removes the redundant type definition of observables, since we didn't have any API for a single observable instance yet.

Fixes #2550
Fixes #2551
Fixes #2553

Short description of the changes

• Add type ObservableCallback to reduce the duplication of the definition of the callback.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• Followed the style guidelines of this project
• Documentation has been updated

[codecov]

Codecov Report

Merging #2687 (fe4c19e) into main (0e9f8af) will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2687      +/-   ##
==========================================
+ Coverage   92.52%   92.57%   +0.04%     
==========================================
  Files         151      151              
  Lines        5283     5276       -7     
  Branches     1116     1116              
==========================================
- Hits         4888     4884       -4     
+ Misses        395      392       -3     

Impacted Files	Coverage Δ
...ages/opentelemetry-api-metrics/src/types/Metric.ts	100.00% <0.00%> (ø)
...ackages/opentelemetry-api-metrics/src/NoopMeter.ts	72.72% <0.00%> (+3.76%)	⬆️

[dyladan]

I'm fine with the name if you are. We can change it in a follow-up if we need