deprecate application_span (traces_spanmetrics_*) metric feature #2923

currently OBI has 2 ways of exporting "spanmetrics"

1. The application_span feature (FeatureSpanLegacy) which emits traces_spanmetrics_* style metrics (grafana convention)
2. The application_span_otel feature (FeatureSpanOTel), which emits traces_span_metrics_* style metrics (otel spanmetrics connector convention)

I would say it makes sense for OBI to have a singular otel compliant spanmetrics feature which it will declare and state in its telemetry schema, so i would strive to keep the application_span_otel feature and deprecate (and maybe eventually remove) application_span feature