Reporter for OTel Metrics

# Description
Agents provide a metrics reporter that sends the metrics from metric registry provided by the OTel metrics SDK to APM Server.

The specifics on how the implementation looks like may be agent specific but the goal is that users have as little to configure as possible.

Unlike our OTel tracing bridge, metrics will not be implemented as a bridge that translates the OTel API calls into calls to the internal metrics registry. Instead, we rely on the OTel metrics SDK to provide the implementation for the OTel metrics API. Agents will only provide a custom reporter that may be registered automatically (for languages that allow for instrumentation) or programmatically.

## Spec Issue
- [ ] https://github.com/elastic/apm/issues/692

## Agent Issues
- [ ] https://github.com/elastic/apm-agent-java/issues/2810
- [ ] https://github.com/elastic/apm-agent-dotnet/issues/1843
- [ ] https://github.com/elastic/apm-agent-nodejs/issues/2954
- [ ] https://github.com/elastic/apm-agent-python/issues/1649
- [ ] https://github.com/elastic/apm-agent-go/issues/1328

Start with the linked spec issue, https://github.com/elastic/apm/issues/692, then compare the agent issues for Java, .NET, Node.js, Python, and Go. Define how each agent-specific reporter uses the OTel metrics SDK and sends registry metrics to APM Server, with automatic or programmatic registration and minimal configuration as the completion criteria.

Written by the indexing model from the issue text.