[SDTEST-409] Telemetry metrics data model

[anmarchenko]

What does this PR do?
This PR is a first step towards full metrics support for internal Datadog telemetry.

It handles data model for the future metrics implementation:

• Adds generate-metrics and distributions events to telemetry
• Adds Ruby classes for every metric type we are going to support

This PR does not handle (subject for subsequent PRs):

• metrics buffering in memory
• multi-threaded synchronization
• sending metrics events to the backend

Motivation:
Add telemetry metrics support in bite-sized and digestible fashion.

Additional Notes:
Pay attention to #track method in each of the metrics implementations: this is the aggregations logic that is different for every metric type.

How to test the change?
Only unit tests currently.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 99.13793% with 2 lines in your changes missing coverage. Please review.

Project coverage is 98.11%. Comparing base (154f72e) to head (a7974f2).

Files	Patch %	Lines
lib/datadog/core/telemetry/event.rb	85.71%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #3734    +/-   ##
========================================
  Coverage   98.10%   98.11%            
========================================
  Files        1227     1229     +2     
  Lines       73014    73246   +232     
  Branches     3508     3515     +7     
========================================
+ Hits        71633    71864   +231     
- Misses       1381     1382     +1     

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[p-datadog]

I read the diff and I have no objections, though I am not familiar with the product enough to leave a checkmark at this time.