Fix build warnings

[marcalff]

Fixes #1611

Changes

Fix build warnings reported by gcc 10, with:
-Wall -Werror
-Wextra-semi
-Werror=unused-parameter
-Werror=undef

For significant contributions please make sure you have completed the following items:

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

[codecov]

Codecov Report

Merging #1613 (1c83594) into main (74f0ac1) will decrease coverage by 0.03%.
The diff coverage is 70.50%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1613      +/-   ##
==========================================
- Coverage   85.24%   85.22%   -0.02%     
==========================================
  Files         156      156              
  Lines        4978     4978              
==========================================
- Hits         4243     4242       -1     
- Misses        735      736       +1     

Impacted Files	Coverage Δ
...elemetry/context/propagation/text_map_propagator.h	50.00% <0.00%> (ø)
api/include/opentelemetry/nostd/string_view.h	98.04% <ø> (ø)
api/include/opentelemetry/trace/noop.h	93.11% <ø> (ø)
...ude/opentelemetry/trace/span_context_kv_iterable.h	100.00% <ø> (ø)
...nclude/opentelemetry/ext/http/client/http_client.h	93.34% <ø> (ø)
ext/src/http/client/curl/http_operation_curl.cc	76.03% <ø> (ø)
...ry/sdk/metrics/aggregation/histogram_aggregation.h	90.00% <0.00%> (ø)
...ry/sdk/metrics/aggregation/lastvalue_aggregation.h	0.00% <0.00%> (ø)
...elemetry/sdk/metrics/aggregation/sum_aggregation.h	0.00% <0.00%> (ø)
...telemetry/sdk/metrics/state/async_metric_storage.h	85.30% <ø> (ø)
... and 23 more

[lalitb]

Looks good to me, but as it's bigger cleanup, will wait for one more approval :). Thanks for taking this.

[owent]

LGTM after the dead codes removed.

[marcalff]

The DocFX check build failed due to this:

The install of docfx was NOT successful.
docfx not installed. An error occurred during installation:
 The remote server returned an error: (500) Internal Server Error. Internal Server Error

which looks like a transient error unrelated to the change.

@lalitb , @ThomsonTan

This PR is ready for merge now.

[esigo]

LGTM
Thanks for the fix :)

[esigo]

related to #249