Fixed many remaining compiler warnings

[marcalff]

Fixes #249

Changes

Fixed trivial warnings reported by:

• gcc
• clang
• msvc

Elasticsearch:

• fixed incomplete error handling in switch case
• reworded error logs

protobuf:

• silenced warnings for clang in protobuf_include_prefix.h

thrift:

• silenced warnings for gcc in thrift_include_prefix.h / thrift_include_suffix.h
• silenced warnings for gcc in CMakeList.txt, for thrift-gen generated code.

gtest:

• silenced warnings for gcc in unit tests using GTest TYPED_TEST()

metrics:

• removed dead code, GetInstrumentClass()

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #1677 (7e56cd9) into main (99f2ba4) will increase coverage by 0.07%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1677      +/-   ##
==========================================
+ Coverage   85.76%   85.82%   +0.07%     
==========================================
  Files         170      169       -1     
  Lines        5137     5133       -4     
==========================================
  Hits         4405     4405              
+ Misses        732      728       -4     

Impacted Files	Coverage Δ
...try/nostd/internal/absl/types/bad_variant_access.h	100.00% <ø> (ø)
...k/metrics/exemplar/fixed_size_exemplar_reservoir.h	71.43% <100.00%> (ø)
...dk/metrics/exemplar/histogram_exemplar_reservoir.h	80.00% <100.00%> (ø)
...ry/sdk/metrics/exemplar/with_trace_sample_filter.h	100.00% <100.00%> (ø)

[lalitb]

nicely done. Thanks for the effort :)