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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

Alternate Designs

Possible Drawbacks

Verification Process

Additional Notes

Release Notes

Review checklist (to be filled by reviewers)

• Feature or bug fix MUST have appropriate tests (unit, integration, etc...)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have one changelog/ label attached. If applicable it should have the backward-incompatible label attached.
• PR should not have do-not-merge/ label attached.
• If Applicable, issue must have kind/ and severity/ labels attached at least.