Add sources reachability metrics

[SuperQ]

Compute two reachability metrics from the "Reachability" bitmask.

• Count the number of 1s in the bitmask as the polling success ratio.
• Expose the right most bit as the "last reach success"

Fixes: #75

[SuperQ]

👓 @hhoffstaette

[hhoffstaette]

First of all thanks a lot 🙌
When I filed the issue I wasn't really sure if this made sense at all, but it looks like the two derived values will be useful.
Give me a minute to build & test. 🚧

[hhoffstaette]

So this looks like it's working:

All in sync:

$curl -s http://localhost:9123/metrics | grep -v '^#' | grep reach
chrony_sources_reachability_ratio{source_address="192.168.100.200",source_name="bifrost,bifrost.applied-asynchrony.com"} 1
chrony_sources_reachability_success{source_address="192.168.100.200",source_name="bifrost,bifrost.applied-asynchrony.com"} 1

Pulled network cable and sent bursts to trigger failing updates:

$curl -s http://localhost:9123/metrics | grep -v '^#' | grep reach
chrony_sources_reachability_ratio{source_address="192.168.100.200",source_name="bifrost,bifrost.applied-asynchrony.com"} 0.625
chrony_sources_reachability_success{source_address="192.168.100.200",source_name="bifrost,bifrost.applied-asynchrony.com"} 0

Neat! 👍

[hhoffstaette]

Works for me!