[headerssetter] Update docs to clarify how to use "from_context" (ope…

…n-telemetry#31283)

This clarifies potential confusion about how to access metadata using
the `from_context` configuration argument.

A few months ago I submitted open-telemetry#27465 to remove an obsolete warning, but
then a user
[asked](open-telemetry@d50e094#commitcomment-131957576)
about why their `from_context` isn't working properly with a batch
processor. I agree that it's not obvious, so I hope this PR clarifies it
enough.

I did not try running the update example, so I hope it works :) 

cc @jpkrohling

extension/headerssetterextension/README.md

@@ -42,6 +42,11 @@ The following settings are required:
 
 The `value` and `from_context` properties are mutually exclusive.
 
+In order for `from_context` to work, other components in the pipeline also need to be configured appropriately:
+* If a [batch processor][batch-processor] is present in the pipeline, it must be configured to [preserve client metadata][batch-processor-preserve-metadata]. 
+ Add the value which `from_context` needs to the `metadata_keys` of the batch processor.
+* Receivers must be configured with `include_metadata: true` so that metadata keys are available to the pipeline.
+
 #### Configuration Example
 
 ```yaml
@@ -67,7 +72,10 @@ receivers:
  include_metadata: true
 
 processors:
- nop:
+ batch:
+ # Preserve the tenant-id metadata.
+ metadata_keys:
+ - tenant_id
 
 exporters:
  loki:
@@ -84,10 +92,13 @@ service:
  pipelines:
  traces:
  receivers: [ otlp ]
- processors: [ nop ]
+ processors: [ batch ]
  exporters: [ loki ]
 ```
 
+[batch-processor]: https://github.com/open-telemetry/opentelemetry-collector/tree/main/processor/batchprocessor/README.md
+[batch-processor-preserve-metadata]: https://github.com/open-telemetry/opentelemetry-collector/tree/main/processor/batchprocessor/README.md#batching-and-client-metadata
+
 [alpha]: https://github.com/open-telemetry/opentelemetry-collector#alpha
 [contrib]: https://github.com/open-telemetry/opentelemetry-collector-releases/tree/main/distributions/otelcol-contrib
 [Mimir]: https://grafana.com/oss/mimir/