[APM] Document OTel infrastructure tab support (#5715)

## Summary

Updates the APM Infrastructure tab docs to reflect new behavior for
OTel-instrumented services (available in 9.4):

- **Previously**, the Infrastructure tab only showed infrastructure
observed by Metricbeat/Elastic Agent.
- **Now**, for OTel-instrumented services, the tab exclusively shows
OTel-observed hosts, pods, and containers, with different click-through
destinations (Hosts UI for hosts; Metrics in Discover for containers and
pods).

Changes:
- `solutions/observability/apm/infrastructure.md`: Split into Elastic
APM and OTel sections; added links to destination UIs.
- `troubleshoot/observability/apm/common-problems.md`: Added
OTel-specific guidance to the "No data in infrastructure tab"
troubleshooting section.

Relates to: elastic/kibana#252188
Closes: #5511

## Generative AI disclosure

1. Did you use a generative AI (GenAI) tool to assist in creating this
contribution?
- [x] Yes  
- [ ] No  

2. If you answered "Yes" to the previous question, please specify the
tool(s) and model(s) used (e.g., Google Gemini, OpenAI ChatGPT-4, etc.).

Tool(s) and model(s) used: Claude Code (claude-sonnet-4-6)

Author: alexandra5000

solutions/observability/apm/infrastructure.md

@@ -3,8 +3,8 @@ mapped_pages:
   - https://www.elastic.co/guide/en/observability/current/apm-infrastructure.html
   - https://www.elastic.co/guide/en/serverless/current/observability-apm-infrastructure.html
 applies_to:
-  stack: ga
-  serverless: ga
+  stack: beta
+  serverless: beta
 products:
   - id: observability
   - id: apm
@@ -13,28 +13,36 @@ products:
 
 # Infrastructure [observability-apm-infrastructure]
 
-::::{important}
+The **Infrastructure** tab provides information about the containers, pods, and hosts that the selected service is linked to. The data sources and navigation behavior depend on whether the service is instrumented with [Elastic {{product.apm}}](#observability-apm-infrastructure-elastic-apm) or [OpenTelemetry (OTel)](#observability-apm-infrastructure-otel).
 
-The Applications UI Infrastructure functionality is in beta and is subject to change. The design and code is less mature than official generally available features and is being provided as-is with no warranties.
+IT ops and software reliability engineers (SREs) can use this tab to quickly find a service’s underlying infrastructure resources when debugging a problem. Knowing what infrastructure is related to a service allows you to remediate issues by restarting, killing hanging instances, changing configuration, rolling back deployments, scaling up, scaling out, and so on.
+
+::::{tip}
+**Why is the infrastructure tab empty?**
 
+If there is no data in the Application UI’s infrastructure tab for a selected service, you can read more about why this happens and how to fix it in the [troubleshooting docs](/troubleshoot/observability/apm/common-problems.md#troubleshooting-apm-infra-data).
 ::::
 
-The **Infrastructure** tab provides information about the containers, pods, and hosts that the selected service is linked to.
+## Elastic {{product.apm}}-instrumented services [observability-apm-infrastructure-elastic-apm]
+
+For services instrumented with Elastic {{product.apm}}, the tab uses the following data sources:
 
-* **Pods**: Uses the `kubernetes.pod.name` from the [APM metrics data streams](/solutions/observability/apm/metrics.md).
-* **Containers**: Uses the `container.id` from the [APM metrics data streams](/solutions/observability/apm/metrics.md).
-* **Hosts**: If the application is containerized—if the APM metrics documents include `container.id`—the `host.name` is used from the infrastructure data streams (filtered by `container.id`). If not, `host.hostname` is used from the APM metrics data streams.
+* **Pods**: Uses the `kubernetes.pod.name` from the [{{product.apm}} metrics data streams](/solutions/observability/apm/metrics.md).
+* **Containers**: Uses the `container.id` from the [{{product.apm}} metrics data streams](/solutions/observability/apm/metrics.md).
+* **Hosts**: If the application is containerized—if the {{product.apm}} metrics documents include `container.id`—the `host.name` is used from the infrastructure data streams (filtered by `container.id`). If not, `host.hostname` is used from the {{product.apm}} metrics data streams.
 
 :::{image} /solutions/images/serverless-infra.png
 :alt: Example view of the Infrastructure tab in the Applications UI
 :screenshot:
 :::
 
-IT ops and software reliability engineers (SREs) can use this tab to quickly find a service’s underlying infrastructure resources when debugging a problem. Knowing what infrastructure is related to a service allows you to remediate issues by restarting, killing hanging instances, changing configuration, rolling back deployments, scaling up, scaling out, and so on.
+## OTel-instrumented services [observability-apm-infrastructure-otel]
+```{applies_to}
+stack: ga 9.4
+serverless: ga
+```
 
-::::{tip}
-**Why is the infrastructure tab empty?**
-
-If there is no data in the Application UI’s infrastructure tab for a selected service, you can read more about why this happens and how to fix it in the [troubleshooting docs](/troubleshoot/observability/apm/common-problems.md#troubleshooting-apm-infra-data).
+For services instrumented with OpenTelemetry, the tab exclusively shows OTel-observed infrastructure. Click-through destinations differ by resource type:
 
-::::
+* **Hosts**: Links to the [**Hosts**](/solutions/observability/infra-and-hosts/analyze-compare-hosts.md) UI, which supports OpenTelemetry and its semantic conventions.
+* **Containers** and **Pods**: Link to [**Metrics** in Discover](/solutions/observability/infra-and-hosts/discover-metrics.md), as the Containers and Pods UIs do not yet support OTel semantic conventions.