Move old metrics implementation to different directory, and rename targets to *_deprecated

[lalitb]

Changes

As the effort required to update our existing metrics SDK implementation as per new specs is huge, it's easier to re-write it from the ground-up ( #1033 ). This means we could

• Either remove the old implementation, which breaks any customers who would be using it, and leave them with no alternative till the new implementation is ready.
• Or let both old and new implementation exists together for some time till new implementation is ready. The old implementation exists under the feature flag WITH_METRICS_PREVIEW. And the new implementation would be active/build only if the feature flag is disabled.

We discussed both approaches in the community meeting and agreed on second. This PR makes necessary changes for it, basically

• Move the old implementation to */_metrics/* directory, and new development continue under */metrics/*.
• Rename cmake and bazel target for old implementation to opentelemetry_metrics_deprecated, prometheus_exporter_deprecated

ps. - We can also use this PR on discussing both approaches if required. The first approach does provide cleaner repo and easy maintainability.

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 #1053 (8aa27a4) into main (14ec785) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1053   +/-   ##
=======================================
  Coverage   94.85%   94.85%           
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  Files         151      151           
  Lines        5971     5971           
=======================================
  Hits         5663     5663           
  Misses        308      308