Exclude header files for metrics from header-only cmake targets

[lalitb]

Changes

If metrics is disabled during cmake build ( i.e, WITH_METRICS_PREVIEW=OFF ), the two header-only targets - opentelemetry-api and opentelemetry-sdk still contain the metrics-related header files. This should be removed.

After this fix:

$ cmake .. -DBUILD_TESTING=OFF -DWITH_METRICS_PREVIEW=OFF && make && make install
...
$ ls /usr/local/include/opentelemetry/
baggage  common  config.h  context  detail  exporters  ext  logs  nostd  plugin  sdk  std  trace  version.h
$ ls /usr/local/include/opentelemetry/sdk/
common  instrumentationlibrary  logs  resource  trace  version
$
$ cmake .. -DBUILD_TESTING=OFF -DWITH_METRICS_PREVIEW=ON && make && make install
...
$ ls /usr/local/include/opentelemetry/
baggage  common  config.h  context  detail  exporters  ext  logs  metrics  nostd  plugin  sdk  std  trace  version.h
$ ls /usr/local/include/opentelemetry/sdk/
common  instrumentationlibrary  logs  metrics  resource  trace  version
$ 

Please provide a brief description of the changes here.

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 #806 (95fd94c) into main (2734b78) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #806   +/-   ##
=======================================
  Coverage   96.00%   96.00%           
=======================================
  Files         176      176           
  Lines        7183     7183           
=======================================
  Hits         6896     6896           
  Misses        287      287