Adjust HTTP client configuration and batch HTTP metrics

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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.

[ ] 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.

jenkinsci / datadog-plugin