This patch changes several internal packages to receive and pass tracing
annotations, making use of the new tracing library, so we can have
better debugging information.
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 adds configuration to integrate with readthedocs.org, using
mkdocs as rendering engine.
It also does minor documentation updates, to tidy some things up
(clearer titles, move some documentation from the root into docs/, etc).
