Add a metric to keep track of the discarded samples

When a profile is ingested, Pyroscope discards invalid stack trace samples in [Normalize](https://github.com/grafana/pyroscope/blob/457d03401c5f0807afe5a4ae614992c37f2d9a9c/pkg/pprof/pprof.go#L357). It would be useful to track these discarded samples as a metric.

We already have `pyroscope_discarded_bytes_total` and `pyroscope_discarded_samples_total` which are updated when an entire profile is rejected (e.g., due to an invalid profile, a limit violation, or a relabel rule).

See https://github.com/grafana/pyroscope/pull/3955#discussion_r1973542611