[SDTEST-116] HTTP transport metrics and minor telemetry tweaks

[anmarchenko]

What does this PR do? Adds the following metrics to the internal library telemetry:

• events_enqueued_for_serialization
• endpoint_payload.requests
• endpoint_payload.requests_ms
• endpoint_payload.requests_errors
• endpoint_payload.bytes
• endpoint_payload.events_count
• endpoint_payload.events_serialization_ms
• endpoint_payload.dropped

How to test the change? Unit tests are provided, tested on staging and confirmed that metrics appear

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.86610% with 11 lines in your changes missing coverage. Please review.

Project coverage is 98.84%. Comparing base (3d14714) to head (8034e41).

Files	Patch %	Lines
...ansport/adapters/telemetry_webmock_safe_adapter.rb	71.42%	4 Missing :warning:
lib/datadog/ci/transport/adapters/net.rb	88.00%	3 Missing :warning:
lib/datadog/ci/transport/http.rb	86.66%	2 Missing :warning:
lib/datadog/ci/configuration/components.rb	92.85%	1 Missing :warning:
...b/datadog/ci/transport/event_platform_transport.rb	95.83%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #201 +/- ## ========================================== - Coverage 98.88% 98.84% -0.04% ========================================== Files 242 247 +5 Lines 10837 11117 +280 Branches 489 502 +13 ========================================== + Hits 10716 10989 +273 - Misses 121 128 +7 ```

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