Run expconfigfetcher only when user has proper auth

It is useless to run this binary when gcert creds aren't available, so
adding a check for gcert creds before calling this binary. Also modified
metrics to capture missing gcert.

Bug: b/237938234
Change-Id: I94b5aa2d53c4e5bb1a5fdc913a33ef5bae965d32
diff --git a/ui/metrics/metrics_proto/metrics.proto b/ui/metrics/metrics_proto/metrics.proto
index 51dd523..4f8fe7f 100644
--- a/ui/metrics/metrics_proto/metrics.proto
+++ b/ui/metrics/metrics_proto/metrics.proto
@@ -251,6 +251,7 @@
     NO_CONFIG = 0;
     CONFIG = 1;
     ERROR = 2;
+    MISSING_GCERT = 3;
   }
   // The result of the call to expconfigfetcher
   // NO_CONFIG - Not part of experiment