Prometheus metric exporter (#378)

prometheus-exporter: initial commit

Author: hectorhdzg

tox.ini

@@ -44,6 +44,10 @@ envlist =
     ; opentelemetry-ext-mysql
     py3{4,5,6,7,8}-test-ext-mysql
     pypy3-test-ext-mysql
+    
+    ; opentelemetry-ext-prometheus
+    py3{4,5,6,7,8}-test-ext-prometheus
+    pypy3-test-ext-prometheus
 
     ; opentelemetry-ext-psycopg2
     py3{4,5,6,7,8}-test-ext-psycopg2
@@ -99,6 +103,7 @@ changedir =
   test-ext-jaeger: ext/opentelemetry-ext-jaeger/tests
   test-ext-dbapi: ext/opentelemetry-ext-dbapi/tests
   test-ext-mysql: ext/opentelemetry-ext-mysql/tests
+  test-ext-prometheus: ext/opentelemetry-ext-prometheus/tests
   test-ext-pymongo: ext/opentelemetry-ext-pymongo/tests
   test-ext-psycopg2: ext/opentelemetry-ext-psycopg2/tests
   test-ext-wsgi: ext/opentelemetry-ext-wsgi/tests
@@ -135,6 +140,8 @@ commands_pre =
   dbapi: pip install {toxinidir}/ext/opentelemetry-ext-dbapi
   mysql: pip install {toxinidir}/ext/opentelemetry-ext-dbapi
   mysql: pip install {toxinidir}/ext/opentelemetry-ext-mysql
+  prometheus: pip install {toxinidir}/opentelemetry-sdk
+  prometheus: pip install {toxinidir}/ext/opentelemetry-ext-prometheus
   pymongo: pip install {toxinidir}/ext/opentelemetry-ext-pymongo
   psycopg2: pip install {toxinidir}/ext/opentelemetry-ext-dbapi
   psycopg2: pip install {toxinidir}/ext/opentelemetry-ext-psycopg2