Port #4280 to serverless (#4296)

* update otel doc with note

* fix copy-paste error

Author: colleenmcginnis

docs/en/serverless/apm-agents/apm-agents-opentelemetry-opentelemetry-native-support.mdx

@@ -133,6 +133,13 @@ java -javaagent:/path/to/opentelemetry-javaagent-all.jar \
     <DocCell>`OTEL_EXPORTER_OTLP_HEADERS`</DocCell>
     <DocCell>
       Authorization header that includes the Elastic APM API key: `"Authorization=ApiKey an_api_key"`.
+      Note the required space between `ApiKey` and `an_api_key`.
+
+      For information on how to format an API key, refer to <DocLink slug="/serverless/observability/apm-keep-data-secure" section="secure-communication-with-apm-agents">Secure communication with APM agents</DocLink>.
+
+      <DocCallOut title="Note">
+        If you are using a version of the Python OpenTelemetry agent _before_ 1.27.0, the content of the header _must_ be URL-encoded. You can use the Python standard library's `urllib.parse.quote` function to encode the content of the header.
+      </DocCallOut>
     </DocCell>
   </DocRow>
   <DocRow>