Telemetry support

[keeganwitt]

I wonder if we should support publishing telemetry metrics. The only metric I can think of offhand is to keep a metric for each rotated file (and whether it succeeded).

[ringerc]

Off the top of my head useful values would include:

• successful rotation count (counter)
• last rotation timestamp (gauge, unix epoch timestamp)
• is cmd running (gauge, bool)
• last exit code of cmd if it has exited since being launched, NaN otherwise (gauge)
• number of times pid in pid_file_name signalling attempted (counter)
• number of times pid in pid_file_name signalling failed with an error (counter)