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To facilitate automation of rolling restarts, configure the prometheus
server to answer readiness and liveness probes. We are 'live' if the
process is running, and we are 'ready' if our component state is
either running or paused (not initializing or stopped).
The prometheus_client library doesn't support this directly, so we need
to handle this ourselves. We could create yet another HTTP server that
each component would need to start, or we could take advantage of the
fact that the prometheus_client is a standard WSGI service and just
wrap it in our own WSGI service that adds the extra endpoints needed.
Since that is far simpler and less resounce intensive, that is what
this change does.
The prometheus_client will actually return the metrics on any path
given to it. In order to reduce the chances of an operator configuring
a liveness probe with a typo (eg '/healthy/ready') and getting the
metrics page served with a 200 response, we restrict the metrics to
only the '/metrics' URI which is what we specified in our documentation,
and also '/' which is very likely accidentally used by users.
Change-Id: I154ca4896b69fd52eda655209480a75c8d7dbac3
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