Metrics API comments after initial PR

[aabmass]

Tracking issue for followups to #1887 (original comment #1887 (review))

• Duplicate meter handling in SDK #2141
  • should we return the same meter or new meter when get_meter() is called with the same params
• Duplicate instrument handling in API #2142
  • Should it be handled at the global level?
  • Which instrument fields are considered non-identifying (description?)
• Log or raise when creating invalid instruments in API #2143
  • For a similar issue in the metrics SIG, someone brought up the idea of a strict MeterProvider (or option to pass) that you can use to strictly raise exceptions for these in unit tests.
  • In the SDK, you should also return a no-op instrument if you only log a warning.
• ProxyMeter must return proxy instruments or users may get stale instruments in the API #2144
• Measurement should be a concrete class in the API not an abc #2150
• Asynchronous.callback() should not be public in the metrics API #2151

[ocelotl]

Do we still need this issue? The metrics dashboard is also tracking the individual tasks listed here 🤷

[lzchen]

Will be deleting this in favour of the individual tasks.