feat: add duration metrics for collectors

[sysadmind]

adds metrics to capture duration of time spent collecting metrics for:

• cluster_health
• clustersettings
• index_stats
• node_stats
• snapshot_stats

this feature was adapted from the caarlos0/elasticsearch_exporter repo.

There seem to be a lot of metrics missing from the README, so I just added the metrics that this PR addresses, but I want to ensure that all metrics are in the README in a future PR.

[SuperQ]

Hmm, typically we add this kind of thing as a gauge. For example in the node_exporter, this is how we handle it.

[sysadmind]

Thanks for the feedback @SuperQ! I have updated the metrics be be gauge types.

[LaurentDumont]

Curious in this one too. I'd like to see if we have a performance issues with queries and it's a bit hard to see right now.

[sysadmind]

@SuperQ We have been talking about a refactor, would these changes be better suited to be included there?

[SuperQ]

This needs a rebase.

[smowky]

this metrics might be really useful, any chance this PR will get processed?