Enable oppportunistic fd counting fast path

Existing slow path takes ~725ms of CPU time on my laptopto count
1 million open files. Compare this to just 0.25ms for the baseline,
when no extra files are open by a Go program:

```
Open files reported: 7
 Gathered metrics in 0.26ms
Open files reported: 1000007
 Gathered metrics in 724.50ms
```

Adding fastpath from Linux v6.2 makes it fast:

```
Open files reported: 6
 Gathered metrics in 0.29ms
Open files reported: 1000006
 Gathered metrics in 0.31ms
```

This is before taking in account any lock contention effects in the kernel
if you try to count files from multiple threads concurrently, which makes
the slow path even slower, burning a lot more CPU in the process. See:

* torvalds/linux@f1f1f2569901

Signed-off-by: Ivan Babrou <[email protected]>

Author: bobrik

proc.go

@@ -237,6 +237,17 @@ func (p Proc) FileDescriptorTargets() ([]string, error) {
 // FileDescriptorsLen returns the number of currently open file descriptors of
 // a process.
 func (p Proc) FileDescriptorsLen() (int, error) {
+	// Use fast path if available (Linux v6.2): https://github.com/torvalds/linux/commit/f1f1f2569901
+	stat, err := os.Stat(p.path("fd"))
+	if err != nil {
+		return 0, err
+	}
+
+	size := stat.Size()
+	if size > 0 {
+		return int(size), nil
+	}
+
 	fds, err := p.fileDescriptors()
 	if err != nil {
 		return 0, err