Clean up push gateway metrics on exit (#805)

This commit adds a cleanup step to the pushgateway application.
This will make it so stale metrics don't hang around.

10_integrations/pushgateway.py

@@ -86,6 +86,7 @@ def run_pushgateway():
     from prometheus_client import (
         CollectorRegistry,
         Counter,
+        delete_from_gateway,
         push_to_gateway,
     )
 
@@ -103,6 +104,16 @@ def init(self):
             registry=self.registry,
         )
 
+    # We must explicitly clean up the metric when the app exits so Prometheus doesn't
+    # keep stale metrics around.
+    @modal.exit()
+    def cleanup(self):
+        delete_from_gateway(
+            self.web_url,
+            job="hello",
+            grouping_key={"instance": self.instance_id},
+        )
+
     @web_endpoint()
     def hello(self):
         self.counter.inc()