SOLR-10654: Prometheus regex cloud pattern fix for core names

[mlbiscoc]

https://issues.apache.org/jira/browse/SOLR-10654

Description

The regex pattern for Solr cloud mode assumed all core names ended with a replica_n[0-9]+ which is incorrect. Some core names should be able to have any single character letter before the numbers.

Solution

Change regex pattern to with . instead of n to match any single character

Tests

testCloudCorePattern and testBadCloudCorePattern to test the regex cloud pattern.

Checklist

Please review the following and check all that apply:

• I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
• I have created a Jira issue and added the issue ID to my pull request title.
• I have given Solr maintainers access to contribute to my PR branch. (optional but recommended, not available for branches on forks living under an organisation)
• I have developed this patch against the main branch.
• I have run ./gradlew check.
• I have added tests for my changes.
• I have added documentation for the Reference Guide