OTel instrumentation of OTLP consumer rejected metrics

For OTLP input, there is currently "monitoring" for `UnsupportedMetricsDropped` in otlp consumer. #156 added partial success support to it, but it returns rejected data points instead of dropped metrics.

We would like to move away from monitoring and have OTel instrumentation of rejected metrics so that all apm-data library users will have access to it.