exporter/prometheusremotewriterexporter: close HTTP response body (#2875)
exporter/prometheusremotewriterexporter: close HTTP response body (#2875)

Fixes a leak in HTTP responses after sending protobuf payloads
to Amazon's endpoints. This change ensures that connections
can be successfully reused, otherwise without invoking .Close()
they won't easily be reused.
Discovered while investigating for anything that could be averse
to scalability of the exporter.

Fixes #2874
open-telemetry/opentelemetry-collector,open-telemetry/opentelemetry-collector,open-telemetry/opentelemetry-collector,open-telemetry/opentelemetry-collector-contrib,open-telemetry/opentelemetry-collector-contrib