receiver/prometheus, exporter/prometheus: recognize SIGHUP to reload configurations as Prometheus server does

[odeke-em]

Prometheus' server recognizes uses a SIGHUP signal sent to it, to reload configurations when they are changed.
Essentially Prometheus says per https://prometheus.io/docs/prometheus/latest/configuration/configuration/
Prometheus can reload configurations by sending SIGHUP or by visiting the /-/reload endpoint with —web.enable-lifecycle flag is enabled

This means that we don't have parity with Prometheus server.

We filed this issue open-telemetry/prometheus-interoperability-spec#3 to implement parity with Prometheus server.

There is an old open issue from August 2019 (2 years ago) to support reload of configurations per open-telemetry/opentelemetry-collector#273 for the entire collector. Perhaps we should also add the general ability for the OpenTelemetry collector to reload every configuration if given a SIGHUP signal.

Please note that this issue isn't a release blocker

[bogdandrutu]

I don't think this is a bug. we are not copying prometheus in this project. We do have other ways to reload config see the ConfigSource and other mechanisms.

[github-actions]

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[github-actions]

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