pyrosope.scrape, pyroscope.write: clarify dropped labels behavior

[korniltsev]

pyroscope.scrape drops all meta labels ( starts with __meta_* )
pyroscope.write drops all private labels (starts with __) , except __name__ and __delta__

Both dropping logic is not documented and an implementation detail.

With the new pyroscope.relabel component, it may be useful in some cases to NOT drop meta labels, so that they can be used in pyroscope.relabel component.

I think we should do one of the following:

• document that pyroscope.scrape drops meta labels
• make the pyroscope.scrape not drop labels

Maybe we should also document pyroscope.write behavior as well

[korniltsev]

cc @marcsanmi @simonswine