Expose internal and plugin metrics via /metrics endpoint

[mostafa]

Plugins can independently expose Prometheus or other types of metrics that can be scraped or sent completely separately from GatewayD, however, it would be nice to have a feature to aggregate all Prometheus metrics in GatewayD, and then expose them on /metrics all at once.

This is my proposal:

• Each plugin creates and exposes their own Prometheus metrics on the /metrics HTTP endpoint on a Unix/TCP socket.

• The configuration will be passed to GatewayD as part of their configs.

• While loading the plugin on startup, GatewayD will register the Unix socket's address or TCP port.

• The merged and relabeled metrics from plugins, along with the ones from GatewayD, will be collected and exposed on the HTTP /metrics endpoints and served on TCP in GatewayD.

• Metrics export gatewayd-plugin-sdk#2