Add naming pattern for Redis HLL metrics to Telemetry guide

What does this MR do?

Adding more context to prefix used.

• g_ prefix is from group, as an event which is tracked for group
• p_ is from project as an event which is tracked for project
• i_ from instance as an event which is tracked for instance
• o_ for other
• a_ all (of g_ p_ i_)

This is a pattern which was initially introduced by manage analytics group, I think make sense to use it further

We might consider adding validations in code. With the mention that we have some naming exceptions.

cc @gitlab-org/growth/telemetry/engineers

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #247472 (closed)