Add pkg/metrics/text, a small Prometheus text-format writer, and use it
for frpc's /metrics. The endpoint lives only on metricsServer and is
covered by its Basic Auth, like the other endpoints there.
The package emits the Go runtime families and, on Linux, the process
families read from /proc. Their names, types and help strings match
client_golang's default collectors, so existing Go runtime dashboards
keep working. It deliberately emits no frp_ business metrics and none of
client_golang's promhttp_ handler metrics.
Writing the subset ourselves keeps client_golang and protobuf out of the
frpc binary; both are still used by frps and by the tests here.
Add a separate HTTP listener for observability, configured through
metricsServer in both frps and frpc. It serves /healthz and, when
pprofEnable is set, the pprof handlers except /debug/pprof/cmdline. It
never serves the dashboard, static assets or the admin API, and it does
not touch the global assets filesystem.
frps also serves /metrics on this listener with both runtime and business
metrics, enabled by metricsServer.port alone. The in-memory collector
still starts only with the dashboard, and the Prometheus collector starts
either with metricsServer or with enablePrometheus plus webServer, so a
configuration that collected nothing before still collects nothing.
The /healthz, /metrics and pprof endpoints on webServer, and
enablePrometheus, keep working but are deprecated and log a hint at
startup. frpc does not serve /metrics yet.
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