Add troubleshooting page for propagating client metadata with EDOT (#2057)

Added a new troubleshooting page that explains why `from_context:
client_metadata` cannot extract application-level attributes like
`project_id` or `tenant`. The page clarifies the distinction between
transport-level and application-level metadata and provides a resolution
using SDK instrumentation.

Author: alexandra5000

troubleshoot/ingest/opentelemetry/edot-collector/metadata.md

@@ -0,0 +1,81 @@
+---
+navigation_title: Collector doesn’t propagate metadata
+description: Learn why the Collector doesn’t extract custom attributes and how to propagate such values using EDOT SDKs.
+applies_to:
+  stack:
+  serverless:
+    observability:
+  product:
+    edot_collector: ga
+products:
+  - id: cloud-serverless
+  - id: observability
+  - id: edot-collector
+---
+
+# EDOT Collector doesn’t propagate client metadata
+
+By default, the Collector only propagates transport-level metadata. If you want the EDOT Collector to propagate metadata like `project_id`, `tenant`, or `environment`, you need a specific SDK instrumentation.
+
+### What is client metadata
+
+In the context of the EDOT, client metadata refers to gRPC metadata or HTTP headers that accompany telemetry data sent by clients (usually SDKs) to the Collector.
+
+For example: 
+
+- Authorization headers
+- Trace propagation headers
+- `user-agent` strings sent by HTTP clients or EDOT SDKs
+
+This is transport-level metadata, and not application-level context.
+
+### What isn’t client metadata
+
+In this context, client metadata does not include:
+
+- Custom attributes like `project_id`, `tenant`, or `environment`
+- Application business logic fields
+- Resource attributes
+
+## Symptoms
+
+Expected labels do not appear in Elastic APM or metric data.
+
+Your collector configuration file, for example `otel-config.yaml`, looks similar to this:
+
+```yaml
+receivers:
+  otlp:
+    protocols:
+      http:
+        endpoint: "0.0.0.0:4318"
+
+processors:
+  resource:
+    attributes:
+      - key: project.id
+        from_context: client_metadata
+        action: insert
+```
+
+This will not work, as the Collector doesn't automatically extract such values from headers.
+
+## Resolution
+
+If you want to propagate customer IDs or project names into spans or metrics, you must instrument this in your code using one of the SDKs.
+
+Use `span.set_attribute` in your application code, where OpenTelemetry spans are created. For example:
+
+```python
+from opentelemetry import trace
+
+tracer = trace.get_tracer(__name__)
+
+with tracer.start_as_current_span("handle_request") as span:
+    span.set_attribute("project.id", get_project_id_from_context())
+```
+
+## Resources
+
+- [gRPC metadata documentation](https://grpc.io/docs/guides/concepts/#metadata)
+- [OpenTelemetry Protocol (OTLP) overview](https://opentelemetry.io/docs/specs/otlp/)

troubleshoot/ingest/opentelemetry/toc.yml

@@ -5,6 +5,7 @@ toc:
   - file: edot-collector/index.md
     children:
       - file: edot-collector/collector-oomkilled.md
+      - file: edot-collector/metadata.md
   - file: edot-sdks/index.md
     children:
       - file: edot-sdks/dotnet/index.md