docs(attributesprocessor): Mention include_metadata is needed to get …

…HTTP/gRPC headers (#35499)

**Description:** 
In the README of attributesprocessor, where it is explain that the user
can use `from_context: metadata.my-header` to get the value of the
HTTP/gRPC header `my-header`, mention that the user should also set
`include_metadata: true` in the receiver configuration, otherwise the
headers are not added to the metadata (and therefore cannot be read by
attributesprocessor).

processor/attributesprocessor/README.md

@@ -60,7 +60,8 @@ For the actions `insert`, `update` and `upsert`,
   action: {insert, update, upsert}
   # FromContext specifies the context value to use to populate the attribute value. 
   # If the key is prefixed with `metadata.`, the values are searched
-  # in the receiver's transport protocol additional information like gRPC Metadata or HTTP Headers. 
+  # in the receiver's transport protocol additional information like gRPC Metadata or HTTP Headers
+  # (be sure to set `include_metadata: true` on the receiver).
   # If the key is prefixed with `auth.`, the values are searched
   # in the authentication information set by the server authenticator.
   # Refer to the server authenticator's documentation part of your pipeline for more information about which attributes are available.