Metrics: expose the number of bytes sent by Trigger Engine

[eddbbt]

Add telemetry in order to record the cumulative amount of bytes from triggers payload, resolve #896 Add new astarte.trigger_engine.http_action_executed.payload_bytes metric Change how triggers are handled to provide accurate metric logging for number of retries and payload size, both are cumulative. Payload size is recorded even in case of http error (for example, when configured API returns 500)

[codecov[bot]]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (595adbd) 68.33% compared to head (f76e555) 68.31%.

Files	Patch %	Lines
...gine/lib/astarte_trigger_engine/events_consumer.ex	0.00%	7 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #899 +/- ## ========================================== - Coverage 68.33% 68.31% -0.02% ========================================== Files 270 270 Lines 7046 7048 +2 ========================================== Hits 4815 4815 - Misses 2231 2233 +2 ```

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