The openmetrics proposed standard says we should use the
`application/openmetrics-text` content type when exporting the metrics.
Currently we use `text/plain` for backwards compatibility with
Prometheus, but the new content type is apparently supported since 2018,
so it should be safe to update to match the current proposed standard.
This patch makes chasquid's monitoring server expose an OpenMetrics
metrics endpoint.
It adds a new package "expvarom" which implements an HTTP handler that
exports expvar variables in the OpenMetrics text format.
Then, the handler is registered by the monitoring server at /metrics
(where most things expect it to be).
The existing exported variables are also extended with descriptions,
which is optional, but improves the readability of the metrics.
