Document that Prometheus exporter does not work with multiprocessing (#4711)

* Document multiprocessing environments are not supported

* Rephrase as limitation by design

* Move documentation to Promtheus documentation folder

* Add documentation in source code README as well

* Link tracking issue.

* Make underscores same length as title

---------

Co-authored-by: Riccardo Magliocchetti <[email protected]>

Author: bastbu

docs/exporter/prometheus/prometheus.rst

@@ -47,6 +47,11 @@ The following environment variables are supported:
 * ``OTEL_EXPORTER_PROMETHEUS_HOST`` (default: "localhost"): The host to bind to
 * ``OTEL_EXPORTER_PROMETHEUS_PORT`` (default: 9464): The port to bind to
 
+Limitations
+-----------
+
+* No multiprocessing support: The Prometheus exporter is not designed to operate in multiprocessing environments (see `#3747 <https://github.com/open-telemetry/opentelemetry-python/issues/3747>`_).
+
 References
 ----------
 

exporter/opentelemetry-exporter-prometheus/README.rst

@@ -15,6 +15,11 @@ Installation
 
      pip install opentelemetry-exporter-prometheus
 
+Limitations
+-----------
+
+* No multiprocessing support: The Prometheus exporter is not designed to operate in multiprocessing environments (see `#3747 <https://github.com/open-telemetry/opentelemetry-python/issues/3747>`_).
+
 References
 ----------