Add Prometheus gauge for current gRPC client connections

[czarcas7ic]

A node runner mentioned having the number of connections as a Prometheus metric would be helpful for them, so added it here.

Summary

• Adds a grpc_server_connections_current Prometheus gauge that tracks active gRPC client connections
• Uses gRPC's stats.Handler interface (ConnBegin/ConnEnd)
• No new dependencies; uses google.golang.org/grpc/stats (part of grpc-go) and promauto (part of existing prometheus/client_golang)

Changes

• cmd/connstats.go — defines the gauge and a connStatsHandler implementing stats.Handler
• cmd/root.go — wires grpc.StatsHandler(&connStatsHandler{}) into both the TLS and non-TLS grpc.NewServer() calls
• cmd/connstats_test.go — unit tests for increment/decrement and passthrough behavior
• go.mod — go mod tidy promoted two already-present indirect deps to direct

Testing

[LarryRuane]

I can't get this to work in testing. It's probably something in my environment. http://127.0.0.1:9090/query returns "127.0.0.1 refused to connect". I think this is consistent with:

$ curl -I http://localhost:9090
curl: (7) Failed to connect to localhost port 9090 after 0 ms: Could not connect to server
$

[ValarDragon]

Its working on my machine too

@LarryRuane I think your issue is a more general setup issue, e.g. no prometheus running in the first place.

Did you install prometheus?

sudo apt update
sudo apt install prometheus

and then I ask claude to install it

[LarryRuane]

Did you install prometheus?

No, I didn't; I'm sure that's the problem. But I don't really need to run it, I'm fine with merging this since some have asked for it, and it's easy to add. I verified that there is no noticeable difference in performance or memory usage with this PR compared to without it. I ran both tests (go test ./... and ./smoke-test.bash) and there are no failures. I did not test the actual functionality (but I'm sure it's okay).

ACK 20e0785