Test metrics validity when testing Exporter

When using the 'test' button for an exporter, we previously *only*
checked the status code and assumed things were good. It is very possible
for an incorrect endpoint to be registered, so we should ensure that the
registered endpoint produces correct Prometheus metrics.

Right now, we will return a somewhat *developer* like output, but I
expect to make the output more user friendly as we cleanup some of our
internal test URLs.

promgen/views.py

@@ -11,6 +11,8 @@
 from itertools import chain
 
 import prometheus_client
+from prometheus_client.parser import text_string_to_metric_families
+
 import requests
 from django.conf import settings
 from django.contrib import messages
@@ -667,7 +669,16 @@ def query():
                     try:
                         result = future.result()
                         result.raise_for_status()
-                        yield result.url, result.status_code
+                        metrics = list(text_string_to_metric_families(result.text))
+                        yield (
+                            result.url,
+                            {
+                                "status_code": result.status_code,
+                                "metric_count": len(list(metrics)),
+                            },
+                        )
+                    except ValueError as e:
+                        yield result.url, f"Unable to parse metrics: {e}"
                     except requests.ConnectionError as e:
                         logger.warning("Error connecting to server")
                         yield e.request.url, "Error connecting to server"