[Synthetics] filters - remove public so client usage

[dominiqueclarke]

Summary

Relates to #153399

Resolves the following task from the referenced issue

• https://github.com/elastic/kibana/blob/main/x-pack/plugins/synthetics/public/apps/synthetics/components/monitors_page/common/monitor_filters/use_filters.ts#L96

Removes public SO client usage for the fetching monitor filters options, by creating a new GET route for getting the available filters and their count.

Testing

This PR affects filters on the overview and management pages. To test, create a few monitors with various combinations of configurations to filter against, for example monitor type, location, tags, and schedule. Be sure to also create a few project monitors.

Utilize the filters on both pages to test for regression.

[elasticmachine]

Pinging @elastic/uptime (Team:uptime)

[apmmachine]

🤖 GitHub comments

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Just comment with:

• /oblt-deploy : Deploy a Kibana instance using the Observability test environments.
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[shahzad31]

LGTM !!

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: 88cf8a6

Metrics [docs]

Module Count

Fewer modules leads to a faster build time

id	before	after	diff
synthetics	1257	1258	+1

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
synthetics	1.2MB	1.2MB	-719.0B

Unknown metric groups

ESLint disabled line counts

id	before	after	diff
enterpriseSearch	19	21	+2
securitySolution	399	403	+4
total			+6

References to deprecated APIs

id	before	after	diff
synthetics	114	110	-4

Total ESLint disabled count

id	before	after	diff
enterpriseSearch	20	22	+2
securitySolution	479	483	+4
total			+6

History

• 💔 Build #129463 failed eae6d06
• 💔 Build #129406 failed 3a0188a
• 💔 Build #129354 failed 3b6cf86
• 💔 Build #129272 failed aed0af0
• 💔 Build #129044 failed 0035625

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@elasticmachine merge upstream