clj-http-client: metrics prefix is not compatible with graphite metrics filtering

Currently the metrics created by the http-client if provided a MetricRegistry are prefixed with puppetlabs.http-client.experimental. However, for filtering metrics before exporting them to a reporter such as Graphite, pe-trapperkeeper-metrics expects that metrics are prefixed by puppetlabs.<server-id>.

pe-tk-metrics has a metrics-prefix setting that allows for specifying a different prefix for a registry, but then the http-client metrics would need to be in their own metrics registry since other libraries don't have the same prefix.

This ticket involves adding two new client options: metrics-prefix and server-id. If server-id is set, then the metrics prefix is puppetlabs.<server-id>.http-client.experimental. If metrics-prefix is set, then the full prefix is <metrics-prefix>.http-client.experimental. If both are set, then metrics-prefix overrides the server-id, and a warning message is logged.