Add message filter support

[dead-claudia]

The idea is this: pass -f/--message-filter. Its value is a name:pattern pair where name is the filter name and pattern is a POSIX regexp. (When name is repeated, last one wins.)

In the metrics, they're all returned with a filter="name" label. Any name works if it doesn't have a colon, and invalid patterns are dropped from the output (causing metrics to be missed). The lack of metrics can be used as a signal to check the logs, which will indicate the filter is wrong.

This is a simplified version of #10 that doesn't require fussing with extra files.