Document the Prometheus ElasticSearch Exporter

Documenting the prometheus exporter is broken out from https://repo1.dso.mil/platform-one/big-bang/apps/core/elasticsearch-kibana/-/issues/3.

Show the default setup for mounting the credentials of the elastic user but also show how a customer can update that ES_USERNAME & ES_PASSWORD if they wish to not use the elastic user. It is also recommended to utilize an elasticsearch API_KEY for authentication over Username/Password, document the steps to clear out ES_PASSWORD envSecret mount and manually create and specify an Elasticsearch API_KEY: https://github.com/prometheus-community/elasticsearch_exporter#elasticsearch-7x-security-privileges

The location for this documentation is in /docs/prometheus.md