Collect ndt-server ping/pong rtt metrics

[stephen-soltesz]

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As part of investigation by @mattmathis to find a method for instrumenting ndt with application metrics we found that some ping/pong rtt metrics are being generated but not collected during ndt-server measurements.

The rtt values are logged but not archived. https://github.com/m-lab/ndt-server/blob/master/ndt7/receiver/receiver.go#L42

To determine if this functionality is used by all clients and to get a sense of the distribution of these values, we should add prometheus histogram that collects the application rtt metrics. Labels should include: direction, protocol (and exclude monitoring measurements).

Based on the counts, and values, we can get a hint about how many clients support this and whether it may be worth archiving this metric.

[mattmathis]

BTW the echo function is generally part of the base spec every IETF inspired protocol. I doubt you will find any implementations that don't support it.