…lation

This is the first iteration of gathering information from the moment
an application is modified to the moment that the chart is
installed. We're measuring the time it takes Shipper to render the
chart, install it, and report that it has been installed.

… by Prometheus

Metrics controller stores the last modified time of each application
as it is created or updated. It's possible that for some
reasons (e.g. releases not getting rolled out for a long time, the
informers missing events, and other cases that I don't currently know
of) our cache would grow to be too big.

This commit adds code to prevent that from happening, by periodically
cleaning up all the entries that have been added more than 1 hour
ago.

This period is currently defined as a constant. We can later get that
as a command-line parameter if it turns out that we need to modify it
in the deployment.

CRUNTIME-438