flags: respect explicit --listen when metrics-endpoint is set

[gecube]

Problem

Fixes #258.

When a metrics endpoint is configured — either directly via --metrics-endpoint / METRICS_ENDPOINT, or derived from --collector-endpoint — flags/init() unconditionally overwrites the listen address:

if *MetricsEndpoint != nil {
    *ListenAddress = "127.0.0.1:10300"
}

This binds the built-in HTTP server to loopback and silently discards any user-provided --listen / LISTEN. As a result you cannot scrape /metrics directly from the agent while it is also remote-writing, even if you explicitly ask for a routable listen address.

Change

Only apply the loopback default when the listen address is still at its compiled-in default:

if *MetricsEndpoint != nil && *ListenAddress == defaultListenAddress {
    *ListenAddress = "127.0.0.1:10300"
}

• No --listen set → unchanged behavior: loopback (127.0.0.1:10300) in remote-write mode.
• --listen / LISTEN set explicitly → honored, so /metrics can be scraped alongside remote write.

Why this is safe

*flags.ListenAddress is consumed only by the HTTP server (main.go, and the Windows agent). The remote-write path gathers metrics in-process via registry.Gather() (prom/remote_writer.go) — it does not scrape its own socket — so honoring --listen cannot affect the remote-write pipeline. On Windows defaultListenAddress is already 127.0.0.1:10300, so the default-mode behavior there is unchanged too, and an explicit --listen is now honored on Windows as well.

Builds cleanly for GOOS=linux and GOOS=windows; go vet ./flags/ is clean.

[gecube]

Local build & test report

Built and tested in a Debian bullseye container (matching the project Dockerfile) on an arm64 Linux VM (OrbStack), Go 1.24.9, CGO_ENABLED=1, -mod=readonly.

Build

$ go build .
BUILD_OK

Vet

$ go vet ./...
(clean, exit 0)

Tests

$ go test ./...
?   github.com/coroot/coroot-node-agent               [no test files]
ok  github.com/coroot/coroot-node-agent/cgroup        0.013s
ok  github.com/coroot/coroot-node-agent/common        0.002s
ok  github.com/coroot/coroot-node-agent/ebpftracer/l7 0.003s
ok  github.com/coroot/coroot-node-agent/logs          2.416s
ok  github.com/coroot/coroot-node-agent/node          0.003s
ok  github.com/coroot/coroot-node-agent/proc          0.003s
(remaining packages have no test files)
exit 0

All packages build and pass; go vet is clean. Cross-compiles cleanly for GOOS=windows as well.

Note: the flags package has no unit tests, and flags.init() early-returns under go test (the strings.HasSuffix(os.Args[0], ".test") guard), so the resolution logic isn't exercised by the suite. Happy to extract the listen-address resolution into a small testable helper and add a focused unit test if you'd prefer that over the minimal in-place change.