Include optional prometheus installation with pip install nucypher[ursula] and inclusion in Dockerfile

deploy/docker/Dockerfile

@@ -7,6 +7,6 @@ RUN apt install gcc libffi-dev wget -y
 WORKDIR /code
 COPY . /code
 
-RUN pip3 install . -r requirements.txt
+RUN pip3 install .[ursula]
 
 CMD ["/bin/bash"]

setup.py

@@ -115,7 +115,10 @@ DEPLOY_REQUIRES = [
     'twine'
 ]
 
+URSULA_REQUIRES = ['prometheus_client']
+
 EXTRAS = {
+    'ursula': URSULA_REQUIRES,
     'docs': DOCS_REQUIRE,
     'dev': DEV_REQUIRES + DOCS_REQUIRE,
     'benchmark': DEV_REQUIRES + BENCHMARK_REQUIRES