Certain built-in metrics could improve conformity against Prometheus metric naming best practice

[ful09003]

While working on #45, I am noticing that Elasticsearch metrics (presumably others, but I have not checked) could improve their self-descriptiveness by following https://prometheus.io/docs/practices/naming/#metric-names, in particular:

...should have a suffix describing the unit, in plural form. Note that an accumulating count has total as a suffix, in addition to the unit if applicable.

As a concrete example, BulkProcessTime is observed as seconds, currently which suggests a metrics suffix like _seconds could be added.