Add OTEL APM to ES Cluster relationship (staging only)

[vagrawal-newrelic]

Summary

• Adds relationship synthesis rule for OTEL-instrumented APM applications calling Elasticsearch clusters
• Staging override only (.stg.yml) — not promoted to production
• Matches on elasticsearch.cluster.name attribute in Span events to build target ELASTICSEARCHCLUSTER entity GUID

How it works

1. OTEL Java Agent enriches ES spans with elasticsearch.cluster.name (read from ES GET / response)
2. This rule matches those spans and creates a CALLS relationship to the ES Cluster entity
3. Entity GUID is built using FARM_HASH on the cluster name (same as entity identifier)

Conditions matched

• eventType: Span
• instrumentation.provider: opentelemetry
• entity.type: SERVICE or APPLICATION
• db.system: elasticsearch
• elasticsearch.cluster.name: present

Test plan

• Validate in staging with test Java app + ES cluster
• Verify relationship appears in Service Map
• Verify relationship expires after 75 minutes of no data
• Test with cluster name change — new relationship should form

[entityBot]

We don't think these changes require an API Review Board (ARB) review, so it is not required to merge this PR. Note that all API changes do require one: if you believe this PR needs an ARB review, please create the ticket and add the ARB:pending label. Reach #api-review-board in case of doubts.