[DPE-3899, DPE-4173] Try to clear connection pool before relating with COS to avoid TIME_WAIT connections; stabilize exporter tests (#245)

## Issue
#219
#229

We are running into a connection in `TIME_WAIT` when starting
`mysql_router_exporter`. This happens in both the K8s and VM charms. The
guess is that this is related to the connection pool we use in our
tests.

## Solution
Clear the connection pool before relating with COS, re-run exporter
tests 5-10 times, observe if tests have stabilized

Author: shayancanonical

tests/integration/test_exporter.py

@@ -128,6 +128,9 @@ async def test_exporter_endpoint(ops_test: OpsTest) -> None:
     else:
         assert False, "❌ can connect to metrics endpoint without relation with cos"
 
+    # clear connection pool before relating metrics-endpoint which starts mysql_router_exporter
+    http.clear()
+
     logger.info("Relating mysqlrouter with grafana agent")
     await ops_test.model.relate(
         f"{GRAFANA_AGENT_APP_NAME}:grafana-dashboards-consumer",
@@ -174,6 +177,8 @@ async def test_exporter_endpoint(ops_test: OpsTest) -> None:
             else:
                 assert False, "❌ can connect to metrics endpoint without relation with cos"
 
+    http.clear()
+
 
 @pytest.mark.group(1)
 @pytest.mark.abort_on_fail
@@ -228,6 +233,9 @@ async def test_exporter_endpoint_with_tls(ops_test: OpsTest) -> None:
             else:
                 assert False, "❌ can connect to metrics endpoint without relation with cos"
 
+    # clear connection pool before relating metrics-endpoint which starts mysql_router_exporter
+    http.clear()
+
     logger.info("Relating mysqlrouter with grafana agent")
     await ops_test.model.relate(
         f"{GRAFANA_AGENT_APP_NAME}:metrics-endpoint", f"{MYSQL_ROUTER_APP_NAME}:metrics-endpoint"
@@ -295,3 +303,5 @@ async def test_exporter_endpoint_with_tls(ops_test: OpsTest) -> None:
             assert (
                 "Issuer: CN = MySQL_Router_Auto_Generated_CA_Certificate" in issuer
             ), "Expected mysqlrouter autogenerated certificate"
+
+    http.clear()