Adjust HTTP client configuration and batch HTTP metrics

[nikita-tkachenko-datadog]

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What does this PR do?

Makes some adjustments and fixes in the HTTP client component:

• limits the number of parallel connections to any given endpoint (Jetty's default was 64, now it is set to 6 with the possibility of overriding via env vars)
• fixes retry logic to apply retries if no request is received, e.g. if the remote side closes the connection prematurely
• adjusts retry settings (initial retry delay is increased, delay random jitter is increased)
• updates the client to always set User-Agent HTTP header in a centralised manner

Adds logic that allows to send multiple metrics in a single HTTP request.

Description of the Change

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• Feature or bug fix MUST have appropriate tests (unit, integration, etc...)
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[sarah-witt]

nice!

[sarah-witt]

🎉