SOLR-17276 Scrape metrics at fixed rate in prometheus-exporter

[rafalh]

https://issues.apache.org/jira/browse/SOLR-17276

Description

Metrics are not scraped by prometheus-exporter at a fixed rate because it uses scheduleWithFixedDelay API to schedule the scraping proces. Duration of scraping influences the actual rate.

Solution

Make sure Solr metrics are scraped at fixed rate by using scheduleAtFixedRate API.
Note: there will be no overlapping executions, because JavaDoc says: "If any execution of this task takes longer than its period, then subsequent executions may start late, but will not concurrently execute.".

Tests

None

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)
• 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

[github-actions]

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[malliaridis]

I am not eligable to merge any changes, but it looks good at first sight. I have one request and one question in regards to the change:

1. Java docs confirm that the execution will not start concurrently. Is it possible to add a test that confirms that (if none are present)?

2. What happens if the scraping interval is very short and the scraping task takes much longer, and that for a longer period of time? How are the scheduled tasks managed? Would that eventually cause any memory issues over time?

[dsmiley]

A one-liner :-D

+1 makes sense to me!