Added the psutil command with the elastic-apm[flask] installation to avoid metrics error (#1751)

* Added the psutil command with the elastic-apm[flask] installation to avoid metrics error

The psutil is required to push the elasticapm.metrics.sets otherwise, the service is not visible in the APM > Services module.

* Fix pre-commit

* Simplify docs note

---------

Co-authored-by: Colton Myers <[email protected]>

Author: someshwaranM

.pre-commit-config.yaml

@@ -1,6 +1,6 @@
 repos:
 -   repo: https://github.com/pycqa/isort
-    rev: 5.10.1
+    rev: 5.12.0
     hooks:
     -   id: isort
         exclude: "(elasticapm/utils/wrapt/.*|tests/utils/stacks/linenos.py|tests/utils/stacks/linenos2.py|tests/contrib/grpc/grpc_app/.*pb2.*.py)"

docs/flask.asciidoc

@@ -23,6 +23,10 @@ NOTE: If you use Flask with uwsgi, make sure to
 http://uwsgi-docs.readthedocs.org/en/latest/Options.html#enable-threads[enable
 threads].
 
+NOTE: If you see an error log that mentions `psutil not found`, you can install
+`psutil` using `pip install psutil`, or add `psutil` to your `requirements.txt`
+file.
+
 [float]
 [[flask-setup]]
 ==== Setup