New metrics type for microdescriptors

Hi Karsten. I just pushed support for microdescriptors to stem...

https://gitweb.torproject.org/stem.git/commitdiff/23d2332b2514fd9194e64cb1859614014087394b

I'm using the metrics type of 'microdescriptor 1.0'. Would you mind adding it to 'https://metrics.torproject.org/formats.html#descriptortypes'?

Thanks! -Damian