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.
This patch improves the organization of the monitoring index page:
- Include the hostname (both OS and configured) for convenience.
- Round the uptime presentation for readability.
- Add a tiny CSS for consistency with the traces.
- Re-arrange the list of links for readability.
