[WIP] Add plot_metrics for generating etcd metric graphs

[perithompson]

When trying to diagnose issues with etcd, it is sometimes necessary to review metrics provided for Prometheus and other monitoring tools that utilise open metrics. If the cluster does not have metrics it can sometimes be difficult to get access to a graph to review the data in a human-readable way.

This PR adds the ability to plot etcd histograms with the ability to extend to other open telemetry metrics at a later date

[perithompson]

@vladimirvivien do you think we should have an etcd_metrics.go package that we can import instead of lumping all together?

[vladimirvivien]

@perithompson Yes, I think if this is etcd specific, the naming of the Go file (etcd_metrics.go) and the name of the script function (etcd_plot_metrics) should reflect that. That way users would know right away that the functionality applies specifically to etcd. This also gives us a place to put future etcd specific functionalities (not just metrics), such as pinging etcd, query etcd for specific values etc.