Implement new SemConv exporter health metrics

[JonasKunz]

This PR implements the new, experimental exporter health metrics defined in semantic conventions. The other metrics will be added in follow-up PRs.
This PR also already included metric-exporter health metrics which are part of an open semconv PR but I think that PR is close to reaching a consensus.

We already have some experimental health metrics in the SDK, which I intend to eventually replace with the new ones from semantic conventions. I'm envisioning the replacement process as follows:

• Implement the new health metrics in a series of PRs, but have them opt-in and keep reporting the legacy metrics by default
• Add autoconfiguration support to configure the health metrics level: e.g. on, off, legacy and maybe in the future extended for opt-in attributes / metrics
• Switch the default setting from legacy to on, this will be a breaking change
• After some grace period remove support for the legacy metrics

This PR is designed with the eventual removal of support for the legacy metrics in mind:

• The new metrics can be reported easily via the ExporterMetrics class
• An ExporterMetricsAdapter is available which mimics the ExporterMetrics api, but can fallback to reporting the legacy metrics instead

This will allow us to easily remove the legacy metrics later on: Simply replace the usages of ExporterMetricsAdapter with ExporterMetrics and delete the adapter and related code.

[jack-berg]

Hey @JonasKunz give me a heads up when you think this is ready for review. Happy to start with a high level review while its still in draft!

[JonasKunz]

@jack-berg a high level review would be appreciated! I'm happy with the overall structure and would mostly polish from now on, so it would be great to get your feedback before I polish things we'll end up changing anyway.

I've updated the PR comment to give an introduction on how I envision the feature implementation in general.