Add a metric to keep track of the discarded samples

[kolesnikovae]

When a profile is ingested, Pyroscope discards invalid stack trace samples in Normalize. 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 #3955 (comment)