Feat/Add manticore prometheus-exporter and stable release v0.8.0

[slopezz]

3scale API Management product is transitioning from using sphinxsearch to manticoresearch

Sphinx prometheus-exporter seems to not be able to monitor a manticore instance, so this PR adds the official manticoresearch prometheus exporter to prometheus-exporter-operator

• The exporter seems to be built at https://github.com/manticoresoftware/manticoresearch-prometheus, however Github project does not seem to be up-to-date (last commit from 2021, but docker images are being created, last one from November 2023)
• I extracted the latest exporter image 6.2.13.0 from DockerHub https://hub.docker.com/r/manticoresearch/prometheus-exporter/tags
• I extracted the correct deployment settings from the helm chart at https://github.com/manticoresoftware/manticoresearch-helm/tree/b5f18599949c2f4d7ac09b07770038ea0111ca94 (exporter seems to be deployed as a sidecar, the original repo has incorrect exporter port)
• I extracted most of metrics info from https://github.com/manticoresoftware/manticoresearch-prometheus/blob/7d18a343d1fe1ec294847a5556071ad973be4716/src/Exporter.php
• Also, there is already a GitHub issue asking for an official grafana-dashboard and prometheus alerts examples (missing by now) Create an officially maintained Prometheus rules (alerting rules) and Grafana dashboard manticoresoftware/manticoresearch-helm#21

Aside from adding the exporter:

• I created a grafana dashboard from scratch by looking at the metrics, as a kind of start point.
  

• I miss one important metric reporting the status of the manticore instance, the usual up metric provided by any exporter, on that case should be manticore_up (0 -> Down and 1 -> UP), that's why in the example alert I added for ManticoreDown, I used an absent promql query function absent(manticore_uptime_seconds) == 1

/kind feature
/kind release
/priority important-soon
/assign

[3scale-robot]

LGTM label has been added.

Git tree hash: 6d319673d2f0aaab6c4057f4baf449986910b3d9

[slopezz]

/approve

[3scale-robot]

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