Add fgprof pprof profiler (#20005)

fgprof is a sampling Go profiler that allows you to analyze On-CPU as
well as Off-CPU (e.g. I/O) time together.

Go's builtin sampling CPU profiler can only show On-CPU time, but it's
better than fgprof at that. Go also includes tracing profilers that can
analyze I/O, but they can't be combined with the CPU profiler.

fgprof is designed for analyzing applications with mixed I/O and CPU
workloads. This kind of profiling is also known as wall-clock profiling.

Whilst fgprof can cause significant STW latencies in applications with a
lot of goroutines (> 1-10k), these latencies only occur if the profile
is requested - it doesn't cause a delay by simply being available.

The fgprof profile is mounted on
`http://localhost:6060/debug/fgprof?seconds=3`

Signed-off-by: Andrew Thornton <art27@cantab.net>

cmd/web.go

@@ -21,6 +21,7 @@ import (
 	"code.gitea.io/gitea/routers"
 	"code.gitea.io/gitea/routers/install"
 
+	"github.com/felixge/fgprof"
 	"github.com/urfave/cli"
 	ini "gopkg.in/ini.v1"
 )
@@ -145,6 +146,7 @@ func runWeb(ctx *cli.Context) error {
 
 	if setting.EnablePprof {
 		go func() {
+			http.DefaultServeMux.Handle("/debug/fgprof", fgprof.Handler())
 			_, _, finished := process.GetManager().AddTypedContext(context.Background(), "Web: PProf Server", process.SystemProcessType, true)
 			log.Info("Starting pprof server on localhost:6060")
 			log.Info("%v", http.ListenAndServe("localhost:6060", nil))