Support for MonitorSLA policy

[clenchpaign-james-IBM]

What would you like to be added:
The ability to configure MonitorSLA policy on a Gateway or an HTTPRoute or a GRPCRoute.

Why this is needed:
Monitoring Service Level Agreements (SLAs) in a gateway is crucial for ensuring consistent performance and maintaining service quality. It helps identifying potential issues early, allowing for timely corrective actions and preventing significant disruptions. This often includes monitoring availability, response time, error rate, throughput and latency of the upstream backend services.

Implementation:
A MonitorSLAPolicy can be defined and attached to a Gateway or an HTTPRoute or a GRPCRoute. The policy defines a single or multiple SLA criteria for specific time windows. Custom Backends can be configured in the policy where notifications or alerts are sent when SLAs are breached.

I am not sure if this idea already exists or fits well in the Gateway API context. Please provide your suggestions.

[howardjohn]

See #554 for some prior discussion. Not necessarily saying we shouldn't do anything here, but it is a very hard problem

[youngnick]

In particular, I think that the link to open-telemetry/semantic-conventions#1675 is the most relevant discussion.