fix(chart/opensearch): metricsPort and plugins usage info in values.yaml

[eyenx]

Description

Trying to figure out how to expose metrics, it was obvious that there is the need of installing an additional plugin to be able to expose prometheus metrics over the serviceMonitor. Also the default port exposing the metrics is 9200 and not 9600.

This fixes the default metricsPort and adds a comment about the plugin to be used in values.yaml

Issues Resolved

#590

Check List

• Commits are signed per the DCO using --signoff

For any changes to files within Helm chart directories:

• Helm chart version bumped
• Helm chart CHANGELOG.md updated to reflect change

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[peterzhuamazon]

Note: We are in the review of an older PR now so will come back to this a little later.