Add metrics-endpoint to istio-gateway #1029

Job	Run time
Publish Charm / Generate the Charm Matrix	6s
Run Tests / Observability Integration Test (1.25-strict/stable)	10m 23s
Run Tests / Observability Integration Test (1.26-strict/stable)	10m 55s
Run Tests / Integration Test (1.27-strict/stable, integration)	9m 44s
Run Tests / Integration Test (1.27-strict/stable, integration-tls-provider)	9m 18s
Run Tests / Lint Code (pilot)	26s
Run Tests / Unit Test (pilot)	37s
Run Tests / Integration Test (1.27-strict/stable, integration-tls-secret)	9m 40s
Run Tests / Lint Code (gateway)	21s
Run Tests / Unit Test (gateway)	35s
Run Tests / Integration Test (1.30-strict/stable, integration)	9m 43s
Run Tests / Integration Test (1.30-strict/stable, integration-tls-provider)	9m 31s
Run Tests / Integration Test (1.30-strict/stable, integration-tls-secret)	9m 31s
Run Tests / Check libraries (istio-pilot) / Check libraries	31s
Run Tests / Check libraries (istio-gateway) / Check libraries	34s
Run Tests / Check libraries (istio-gateway) / Lint	9s
Run Tests / Check libraries (istio-pilot) / Lint	10s
Publish Charm / Publish Charm (./charms/istio-pilot)	19m 42s
Publish Charm / Publish Charm (./charms/istio-gateway)	2m 32s
	1h 44m 28s

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