[exporter-metrics-otlp-*] investigate moving metrics-specific exporter base to otlp-exporter-base

[pichlermarc]

Description

The OTLP exporter base is currently in @opentelemetry/exporter-metrics-oltp-http which means that all other exporters have to depend on this package which is odd.

@opentelemtry/otlp-exporter-base currently holds all shared configuration code as well as signal specific code, which is hidden behind entry points. Goal of this issue is to discuss with other SIG members if there's push-back to moving the base class there under an @opentelemtry/otlp-exporter-base/metrics entrypoint, and if yes, what the problem behind this is.

This issue is considered done when:

• consensus has been reached about if moving to @opentelemetry/sdk-trace-base is okay
• a follow-up issue has been created to move the base there or to move it elsewhere, if another package was chosen.
  • the follow-up issue has been added to the milestone.