[SDTEST-409] Telemetry metrics support

[anmarchenko]

What does this PR do?
This is the final PR to provide metrics support to internal library telemetry.

Notable changes include:

• Telemetry::Component now has a public API to track metrics (#inc, #dec, #gauge, #rate, #distribution)
• Telemetry::Worker and Telemetry::Component now have shared access to MetricsManager
• new settings are added: DD_TELEMETRY_METRICS_ENABLED to be able to disable sending metrics separately from telemetry, DD_TELEMETRY_METRICS_AGGREGATION_INTERVAL is internal-only setting to change interval to aggregate and send out metrics (10 seconds by default as defined by spec)
• Telemetry::Worker now maintains heartbeat ticker: we aggregate metrics every 10 seconds but we send heartbeat events every 60 seconds, so it takes (heartbeat_interval / metrics_aggregation_interval) ticks for every heartbeat
• Flushing metrics logic is changed: we actually need to flush metric events to an array to be able to send them out in the same pass of Worker#perform - this is needed to be able to flush everything on shutdown

Additional Notes:
Usage in your products:

telemetry = Datadog.send(:components)
telemetry.inc('tracers', 'spans_created', 10, tags: { 'integration_name' => 'pg' })

How to test the change?
Unit tests are provided. Please tell me if we have any product that would like to use it to submit metrics.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.91%. Comparing base (ca006e9) to head (1f01634).
Report is 10 commits behind head on master.

Additional details and impacted files

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  Coverage   97.91%   97.91%            
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  Files        1243     1243            
  Lines       74763    74894   +131     
  Branches     3608     3611     +3     
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+ Hits        73205    73336   +131     
  Misses       1558     1558            

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[marcotc]

Awesome work, @anmarchenko!