Initialize tracer, meter and excluded urls at instrument time

So they are testable

Author: xrmx

instrumentation/opentelemetry-instrumentation-aiohttp-server/src/opentelemetry/instrumentation/aiohttp_server/__init__.py

@@ -113,9 +113,9 @@ async def hello(request):
     HTTP_SERVER_NAME,
 ]
 
-tracer = trace.get_tracer(__name__)
-meter = metrics.get_meter(__name__, __version__)
-_excluded_urls = get_excluded_urls("AIOHTTP_SERVER")
+tracer = None
+meter = None
+_excluded_urls = None
 
 
 def _parse_duration_attrs(req_attrs):
@@ -315,10 +315,16 @@ class AioHttpServerInstrumentor(BaseInstrumentor):
     """
 
     def _instrument(self, **kwargs):
-        # update the excluded urls value at instrument time so we can test it
-        global _excluded_urls  # pylint: disable=global-statement
+        # update global values at instrument time so we can test them
+        global _excluded_urls
         _excluded_urls = get_excluded_urls("AIOHTTP_SERVER")
 
+        global tracer  # pylint: disable=global-statement
+        tracer = trace.get_tracer(__name__)
+
+        global meter  # pylint: disable=global-statement
+        meter = metrics.get_meter(__name__, __version__)
+
         self._original_app = web.Application
         setattr(web, "Application", _InstrumentedApplication)