server: use -server.path-prefix for metrics and debug endpoints

[adamdecaf]

Previously calling Subrouter() would overwrite the instrumentation handlers and they'd be ignored when PathPrefix was specified. This meant one could never get back metrics or debug endpoints.

I verified metrics and api endpoints respond with this change locally, but writing a test didn't work as creating a new Server (via New(..)) mutates global state and prometheus panics on duplicate metric registrations. We could pass in a prometheus registry, but that breaks the public constructor.

func TestInstrumentationPathPrefix(t *testing.T) {
	var cfg Config
	cfg.PathPrefix = "/loki/"
	cfg.HTTPListenPort = 10101
	cfg.RegisterInstrumentation = true
	server, err := New(cfg)
	require.NoError(t, err)

	go server.Run()
	defer server.Shutdown()

	resp, err := http.DefaultClient.Get("http://localhost:10101/loki/metrics")
	if resp != nil && resp.Body != nil {
		resp.Body.Close()
	}
	require.NoError(t, err)
	if resp.StatusCode != http.StatusOK {
		t.Errorf("bogus HTTP status: %v", resp.Status)
	}
}

Related: #144

[adamdecaf]

This does break the idea that /metrics always responds as it's /${prefix}/metrics now.

[bboreham]

Seems legit.