[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 :warning:

Additional details and impacted files

```diff @@ 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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