[Fleet] Add global data tags telemetry

[nchaulet]

Summary

Resolve #184504

Add telemetry for the new global data tags feature

+   "count_with_global_data_tags": 2, // Count of policies using global data tags
+    "avg_number_global_data_tags_per_policy": 3  // Average number of fields per policy

Tests

That change is covered with an automated tests.

That PR will need a follow up PR in telemetry to had the mappings.

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[obltmachine]

🤖 GitHub comments

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Just comment with:

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

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: 66d2579

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Metrics [docs]

✅ unchanged

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@elasticmachine merge upstream

[kpollich]

Code LGTM