Add alert rules to istio-gateway based on the KF093 spec #1124

Job	Run time
Run Tests / Observability Integration Test (1.25-strict/stable)	2m 52s
Publish Charm / Generate the Charm Matrix	3s
Run Tests / Observability Integration Test (1.26-strict/stable)	2m 49s
Run Tests / Integration Test (1.27-strict/stable, integration)	2m 50s
Run Tests / Integration Test (1.27-strict/stable, integration-tls-provider)	2m 50s
Run Tests / Lint Code (pilot)	25s
Run Tests / Integration Test (1.27-strict/stable, integration-tls-secret)	2m 50s
Run Tests / Lint Code (gateway)	20s
Run Tests / Unit Test (pilot)	41s
Run Tests / Integration Test (1.30-strict/stable, integration)	2m 51s
Run Tests / Unit Test (gateway)	32s
Run Tests / Integration Test (1.30-strict/stable, integration-tls-provider)	2m 50s
Run Tests / Integration Test (1.30-strict/stable, integration-tls-secret)	2m 49s
Run Tests / Check libraries (istio-gateway) / Check libraries	26s
Run Tests / Check libraries (istio-gateway) / Lint	11s
Run Tests / Check libraries (istio-pilot) / Check libraries	23s
Run Tests / Check libraries (istio-pilot) / Lint	11s
Publish Charm / Publish Charm (./charms/istio-pilot)	2m 39s
Publish Charm / Publish Charm (./charms/istio-gateway)	2m 41s
	31m 13s

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