Add type label to discarded samples to distinguish between float and native histogram samples

[CharlieTLe]

The DiscardedSamples metric will soon keep track of both discarded float and native histogram samples, but they will be aggregated together without any categorization of what type of sample it is.

It would be useful to know what kind of samples are being discarded while debugging ingestion of native histograms of float value samples.

Originally posted by @yeya24 in #6218 (comment)