Metrics endpoint

[ZoopOTheGoop]

We should add a metrics endpoint that returns something like:

# HELP ubuntu_reportd_requests_total The total number of processed requests
# TYPE ubuntu_reportd_requests_total counter
ubuntu_reportd_requests_total 1234

# HELP ubuntu_reportd_errors_total The total number of errors
# TYPE ubuntu_reportd_errors_total counter
ubuntu_reportd_errors_total 56

# HELP ubuntu_reportd_uptime_seconds The number of seconds the service has been up
# TYPE ubuntu_reportd_uptime_seconds gauge
ubuntu_reportd_uptime_seconds 3600