Improve notification instrumentation (#1335)
Improve notification instrumentation (#1335)

* Improve notification instrumentation

- Add notificationLatencySeconds histogram to
debug duplicate messages. This can help rule out
if duplicate messages are being caused by
excessive latency when sending a notification.

Signed-off-by: stuart nelson <f07588fb19e9aef0215864aeb9ff233a855ee86d@gmail.com>

prometheus/alertmanager,Kellel/alertmanager,GoogleCloudPlatform/alertmanager,GoogleCloudPlatform/alertmanager,GoogleCloudPlatform/alertmanager,prometheus/alertmanager,prometheus/alertmanager,Kellel/alertmanager,Kellel/alertmanager,GoogleCloudPlatform/alertmanager,prometheus/alertmanager