SpanMetricsProcessor for agent instrumentation

Is your feature request related to a problem? Please describe.
Opentelmetry java instrumentation is nicely able to handle the application traces using annotations, a nice feature to have would be to record metrics of the spans using the annotations itself. The metrics would be of the way like span metrics processor does to record the R.E.D metrics.

Describe the solution you'd like
We can use the current span annotations to make a histogram.
Histogram itself will give the counter metrics.
We can define two fields in the annotation, one to disable metrics for a particular method and another to add a gauge for the corresponding method.
The whole feature can be disabled by default and can be enabled on demand by using a configuration option.
Attributes of a span can be used as labels and exceptions can be used as the corresponding error metrics (can be thought more on the same).
These metrics can be exposed using the prometheus metrics exporter.

Describe alternatives you've considered
We have currently been using the span metrics processor at the otel collector end for the same purpose where the traces are being pushed and the metrics are consolidated using the processor.

But these metrics are being impacted by the trace sampling set on client. If we make tracing 100% then it can impact performance. Also most of the times the consolidation can easily be handled at the client end easily and further consolidation can take place at prometheus, like most of the applications run today.

Even if we are using a dummy Span (in case of sampled trace), I believe we can record metrics (I may be wrong here), thus allowing us to not push traces 100% of the times yet getting accurate metrics data.

This would further enhance user experience and allow them to record application metrics without much hassel.

Additional Context
A lot of open source tools are adopting span metrics processor for showing R.E.D metrics along side their traces few examples include

Grafana
Signoz
Jaegar

The issue names no files or tests. Start by reviewing the existing Java span-annotation instrumentation, the span metrics processor approach, and the Prometheus exporter path; clarify where configuration and annotation options belong. Done means an agreed implementation scope covering opt-in metrics, histogram/R.E.D. data, labels, errors, and sampling behavior.