[FEATURE] Add socket activation for the metrics socket

Description

I would like to use systemd socket activation with the metrics socket exposed by SOCI, so that system administrators can decide how they expose this endpoint to services like prometheus

Describe the solution you'd like

I would like soci-snapshotter-grpc to expose a flag or configuration to allow it to use the file descriptors provided by systemd for the metrics endpoint, when the soci-snapshotter-grpc service is activated through a socket.

Describe any alternative solutions/features you've considered

I could use something like socat and redirect traffic with it, however this adds an additional dependency, and that wouldn't be great for container-optimized distributions like Bottlerocket.

Any additional context or information about the feature request

I want to run the SOCI snapshotter in its own network namespace, but keep the metrics stack available in the host network namespace so that I can expose that endpoint to other system services. I can achieve this with socket activation since the socket passed to SOCI will be created in the host network namespace.