Document public metric SDK interfaces to remain stable

[MrAlias]

Resolve #3673

Similar to #2012, document for developers so they are aware things should not change in the SDK.

[CharlieTLe]

Would it make sense to have tests that will fail if there are any new, modified, or removed exported methods?

[MrAlias]

Would it make sense to have tests that will fail if there are any new, modified, or removed exported methods?

Adding test will only add a second place (the interface definitions in the test) where we are not allowed to change anything. I don't see that as very useful.

We could potentially look into using https://pkg.go.dev/golang.org/x/exp/cmd/gorelease to automate all release with a versioning test. It would need more evaluation though.

[MrAlias]

From IRL conversation with @pellared :

• The Aggregation interface: is it neede?
  • Can we remove it and just export the concrete types?
  • The SDK can define its own private version of this interface to reference all the concrete types
  • The Stream will need a way to reference these types. It likely makes sense to change the field to inteface{} type
• The Reader interface may need to be extended based on future changes to the specification.
  • We probably don't want to document the interface cannot change, but instead need a plan for how we will extend it in the future.

[pellared]

Just one comment.

[codecov]

Codecov Report

Merging #4396 (ec06e87) into main (01d64c3) will increase coverage by 0.0%.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main   #4396   +/-   ##
=====================================
  Coverage   81.3%   81.3%           
=====================================
  Files        220     220           
  Lines      17670   17670           
=====================================
+ Hits       14378   14380    +2     
+ Misses      2992    2990    -2     
  Partials     300     300           

Files Changed	Coverage
sdk/metric/reader.go	ø