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

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](https://github.com/digitalocean/firebolt/blob/main/node/elasticsearch/elastic_index_client.go#L176) which suggests a metrics suffix like `_seconds` could be added.