feat: document tagging and enhanced metrics

[kccox]

What has been done?

This PR does not change any code. It is an update of Event Streams documentation with:

• Additional documentation of the use of "ibm_resource_instance" to create Event Streams instances
• New parameters available for provisioning (metrics and kms_key_crn)
• An example of using "ibm_resource_tag" to add an access tag to an Event Streams service instance

Community Note

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Relates N/A

Output from acceptance testing:

No code changes, unit tests for the current Event Streams resources unchanged.