[alloy] Implement a generic Tower metrics service layer, use in Alloy

[dancoombs]

Describe the feature We currently have 2 Tower service layer implementations for metrics:

Incoming RPC

Internal gRPC

We need a 3rd for our new Alloy Provider to track "Internal RPC" metrics.

Instead of re-implementing this logic for a 3rd time, we should build a standard layer that takes a much simpler metrics logger trait impl that can extract method info and do metrics logging. @dancoombs can help with the initial design.