Metrics: expose API calls details in services

[Annopaolo]

Add the total number of bytes exchanged via HTTP to metrics (both sent and received). In general, each API service now exposes four new metrics:

• <service_name>_requests_count
• <service_name>_requests_bytes
• <service_name>_responses_count
• <service_name>_responses_bytes

Values are tagged by realm name, except for Housekeeping API.

In order not to slow down the handling of incoming calls, count and byte size are computed in a different process.

[codecov[bot]]

Codecov Report

Attention: 8 lines in your changes are missing coverage. Please review.

Comparison is base (a882ec1) 68.34% compared to head (64fbeaa) 68.47%.

Files	Patch %	Lines
...ngine_api_web/plug/telemetry_request_count_plug.ex	88.88%	1 Missing :warning:
...gine_api_web/plug/telemetry_response_count_plug.ex	83.33%	1 Missing :warning:
...eping_api_web/plug/telemetry_request_count_plug.ex	85.71%	1 Missing :warning:
...ping_api_web/plug/telemetry_response_count_plug.ex	80.00%	1 Missing :warning:
...iring_api_web/plug/telemetry_request_count_plug.ex	87.50%	1 Missing :warning:
...ring_api_web/plug/telemetry_response_count_plug.ex	83.33%	1 Missing :warning:
...ement_api_web/plug/telemetry_request_count_plug.ex	88.88%	1 Missing :warning:
...ment_api_web/plug/telemetry_response_count_plug.ex	83.33%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #905 +/- ## ========================================== + Coverage 68.34% 68.47% +0.12% ========================================== Files 274 278 +4 Lines 7067 7108 +41 ========================================== + Hits 4830 4867 +37 - Misses 2237 2241 +4 ```

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[Annopaolo]

Superseded by #910.