add PssFreeSnapshot (#115)

## What does this PR do?

Frees the memory that is allocated when PssCaptureSnapshot is called.

## Why is it important?

memory leak

## Checklist

- [x] My code follows the style guidelines of this project
- [x] I have commented my code, particularly in hard-to-understand areas
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] I have added an entry in `CHANGELOG.md`

## Author's Checklist

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- [ ] Written by hand. Need to figure out what needs to happen for "go
generate" to work. Does not work from Mac.

## Related issues

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word "Closes" if merging this should automatically close it.

- Closes #123
- Relates #123
- Requires #123
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- 

## Testing

- run metricbeat 8.11.0 or 8.11.1 on Windows, notice memory consumption
of metricbeat continually climbs
- rebuild metricbeat using this pr
- memory consumption rises in first 10min or so, but is steady after
that

metric/system/process/process_windows.go

@@ -80,26 +80,39 @@ func GetInfoForPid(_ resolve.Resolver, pid int) (ProcState, error) {
 }
 
 func FetchNumThreads(pid int) (int, error) {
-	pHandle, err := syscall.OpenProcess(
+	targetProcessHandle, err := syscall.OpenProcess(
 		xsyswindows.PROCESS_QUERY_INFORMATION,
 		false,
 		uint32(pid))
 	if err != nil {
 		return 0, fmt.Errorf("OpenProcess failed for PID %d: %w", pid, err)
 	}
-	defer syscall.CloseHandle(pHandle)
+	defer syscall.CloseHandle(targetProcessHandle)
+
+	currentProcessHandle, err := syscall.GetCurrentProcess()
+	if err != nil {
+		return 0, fmt.Errorf("GetCurrentProcess failed: %w", err)
+	}
+	// The pseudo handle need not be closed when it is no longer
+	// needed, calling CloseHandle has no effect.  Adding here to
+	// remind us to close any handles we open.
+	defer syscall.CloseHandle(currentProcessHandle)
 
 	var snapshotHandle syscall.Handle
-	err = PssCaptureSnapshot(pHandle, PSSCaptureThreads, 0, &snapshotHandle)
+	err = PssCaptureSnapshot(targetProcessHandle, PSSCaptureThreads, 0, &snapshotHandle)
 	if err != nil {
 		return 0, fmt.Errorf("PssCaptureSnapshot failed: %w", err)
 	}
 
 	info := PssThreadInformation{}
 	buffSize := unsafe.Sizeof(info)
-	err = PssQuerySnapshot(snapshotHandle, PssQueryThreadInformation, &info, uint32(buffSize))
-	if err != nil {
-		return 0, fmt.Errorf("PssQuerySnapshot failed: %w", err)
+	queryErr := PssQuerySnapshot(snapshotHandle, PssQueryThreadInformation, &info, uint32(buffSize))
+	freeErr := PssFreeSnapshot(currentProcessHandle, snapshotHandle)
+	if queryErr != nil || freeErr != nil {
+		//Join discards any nil errors
+		return 0, errors.Join(
+			fmt.Errorf("PssQuerySnapshot failed: %w", queryErr),
+			fmt.Errorf("PssFreeSnapshot failed: %w", freeErr))
 	}
 
 	return int(info.ThreadsCaptured), nil

metric/system/process/syscall_windows.go

@@ -21,6 +21,7 @@ package process
 //  - https://learn.microsoft.com/en-us/previous-versions/windows/desktop/proc_snap/overview-of-process-snapshotting
 // PssCaptureSnapshot docs in https://learn.microsoft.com/en-us/windows/win32/api/processsnapshot/nf-processsnapshot-psscapturesnapshot
 // PssQuerySnapshot docs in https://learn.microsoft.com/en-us/windows/win32/api/processsnapshot/nf-processsnapshot-pssquerysnapshot
+// PssFreeSnapshot docs in https://learn.microsoft.com/en-us/windows/win32/api/processsnapshot/nf-processsnapshot-pssfreesnapshot
 
 // Use golang.org/x/sys/windows/mkwinsyscall instead of adriansr/mksyscall
 // below once https://github.com/golang/go/issues/42373 is fixed.
@@ -29,6 +30,7 @@ package process
 
 //sys PssCaptureSnapshot(processHandle syscall.Handle, captureFlags PSSCaptureFlags, threadContextFlags uint32, snapshotHandle *syscall.Handle) (err error) [failretval!=0] = kernel32.PssCaptureSnapshot
 //sys PssQuerySnapshot(snapshotHandle syscall.Handle, informationClass uint32, buffer *PssThreadInformation, bufferLength uint32) (err error) [failretval!=0] = kernel32.PssQuerySnapshot
+//sys PssFreeSnapshot(processHandle syscall.Handle, snapshotHandle syscall.Handle) (err error) [failretval!=0] = kernel32.PssFreeSnapshot
 
 // The following constants are PssQueryInformationClass as defined on
 // https://learn.microsoft.com/en-us/windows/win32/api/processsnapshot/ne-processsnapshot-pss_query_information_class