Catch KeyError exception in prometheus

[gbregman]

We ran into a case in which a failure in SPDK caused a KeyError exception in the prometheus code which crashed the gateway. I suggest making this change to protect the gateway:

diff --git a/control/prometheus.py b/control/prometheus.py
index 4d3886b..65d2392 100644
--- a/control/prometheus.py
+++ b/control/prometheus.py
@@ -391,7 +391,10 @@ class NVMeOFCollector:
                     str(ns.anagrpid)
                 ], 1)

-            conn_info = self.connections[nqn]
+            try:
+                conn_info = self.connections[nqn]
+            except KeyError:
+                continue
             for conn in conn_info.connections:
                 host_connection_state.add_metric([
                     self.gw_metadata.name,