[DOCS] - Minor updates to OStream Metrics exporter documentation

[lalitb]

Fixes #

Changes

Minor updates to the OStream exporter example documentation

• Add docs for Observable Instrument creation.
• Remove reference to old metrics design docs.
• Mention View creation as optional step, and view with default configuration is created by SDK if missing.
• small indentation changes.

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 #1679 (ed5462c) into main (3ff2ed8) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1679   +/-   ##
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  Coverage   85.82%   85.82%           
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  Files         169      169           
  Lines        5133     5133           
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  Hits         4405     4405           
  Misses        728      728           

[marcalff]

Thanks for adding metrics documentation.

See a few questions / comments.