ext/system-metrics: adding instrumentation to collect system metrics (#652)

Adding an extension to provide users an easy mechanism to collect metrics for their system.

tox.ini

@@ -114,6 +114,11 @@ envlist =
     py3{4,5,6,7,8}-test-ext-redis
     pypy3-test-ext-redis
 
+    ; opentelemetry-ext-system-metrics
+    py3{4,5,6,7,8}-test-ext-system-metrics
+    ; ext-system-metrics intentionally excluded from pypy3
+    ; known limitation: gc.get_count won't work under pypy
+
     ; Coverage is temporarily disabled for pypy3 due to the pytest bug.
     ; pypy3-coverage
 
@@ -162,6 +167,7 @@ changedir =
   test-opentracing-shim: ext/opentelemetry-ext-opentracing-shim/tests
   test-ext-sqlalchemy: ext/opentelemetry-ext-sqlalchemy/tests
   test-ext-redis: ext/opentelemetry-ext-redis/tests
+  test-ext-system-metrics: ext/opentelemetry-ext-system-metrics/tests
 
 commands_pre =
 ; Install without -e to test the actual installation
@@ -220,6 +226,9 @@ commands_pre =
 
   sqlalchemy: pip install {toxinidir}/opentelemetry-auto-instrumentation {toxinidir}/ext/opentelemetry-ext-sqlalchemy
 
+  system-metrics: pip install {toxinidir}/opentelemetry-auto-instrumentation
+  system-metrics: pip install {toxinidir}/ext/opentelemetry-ext-system-metrics[test]
+
 ; In order to get a healthy coverage report,
 ; we have to install packages in editable mode.
   coverage: python {toxinidir}/scripts/eachdist.py install --editable
@@ -317,6 +326,7 @@ commands_pre =
               -e {toxinidir}/ext/opentelemetry-ext-pymysql \
               -e {toxinidir}/ext/opentelemetry-ext-sqlalchemy \
               -e {toxinidir}/ext/opentelemetry-ext-redis \
+              -e {toxinidir}/ext/opentelemetry-ext-system-metrics \
               -e {toxinidir}/ext/opentelemetry-ext-opencensusexporter
   docker-compose up -d
   python check_availability.py