[receiver/hostmetrics] Fix panic in load_scraper_windows when stopping (

open-telemetry#28678)

**Description:** 
Fix a panic when the load scraper for Windows is stopped before being
started. This can happen when the collector fails at startup. In this
case the components are shutdown even if they were not started. This was
encountered in real world usage.

```terminal
2023-10-23T13:13:23.137-0500    info    service@v0.86.0/service.go:170  Starting shutdown...
2023-10-23T13:13:23.138-0500    info    healthcheck/handler.go:132      Health Check state change       {"kind": "extension", "name": "health_check", "status": "unavailable"}
panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xc0000005 code=0x0 addr=0x0 pc=0x30c4028]

goroutine 1 [running]:
github.com/open-telemetry/opentelemetry-collector-contrib/receiver/hostmetricsreceiver/internal/scraper/loadscraper.stopSampling({0x0?, 0x6000103?})
        github.com/open-telemetry/opentelemetry-collector-contrib/receiver/hostmetricsreceiver@v0.86.0/internal/scraper/loadscraper/load_scraper_windows.go:145 +0xc8
github.com/open-telemetry/opentelemetry-collector-contrib/receiver/hostmetricsreceiver/internal/scraper/loadscraper.(*scraper).shutdown(...)
        github.com/open-telemetry/opentelemetry-collector-contrib/receiver/hostmetricsreceiver@v0.86.0/internal/scraper/loadscraper/load_scraper.go:78
go.opentelemetry.io/collector/component.ShutdownFunc.Shutdown(...)
        go.opentelemetry.io/collector/component@v0.86.0/component.go:84
go.opentelemetry.io/collector/receiver/scraperhelper.(*controller).Shutdown(0xc0000c3a40, {0x71b0d50, 0xc00006c0e0})
        go.opentelemetry.io/collector/receiver@v0.86.0/scraperhelper/scrapercontroller.go:149 +0x97
go.opentelemetry.io/collector/service/internal/graph.(*Graph).ShutdownAll(0x0?, {0x71b0d50, 0xc00006c0e0})
        go.opentelemetry.io/collector/service@v0.86.0/internal/graph/graph.go:358 +0xc9
go.opentelemetry.io/collector/service.(*Service).Shutdown(0xc0008373b0, {0x71b0d50, 0xc00006c0e0})
        go.opentelemetry.io/collector/service@v0.86.0/service.go:176 +0xd4
go.opentelemetry.io/collector/otelcol.(*Collector).setupConfigurationComponents(0xc000dc6180, {0x71b0d50, 0xc00006c0e0})        go.opentelemetry.io/collector/otelcol@v0.86.0/collector.go:187 +0x708
go.opentelemetry.io/collector/otelcol.(*Collector).Run(0xc000dc6180, {0x71b0d50, 0xc00006c0e0})
        go.opentelemetry.io/collector/otelcol@v0.86.0/collector.go:221 +0x65
go.opentelemetry.io/collector/otelcol.NewCommand.func1(0xc00229cf00, {0x6636c91?, 0x0?, 0x3?})
        go.opentelemetry.io/collector/otelcol@v0.86.0/command.go:27 +0x96
github.com/spf13/cobra.(*Command).execute(0xc00229cf00, {0xc0000ac050, 0x0, 0x3})
        github.com/spf13/cobra@v1.7.0/command.go:940 +0x862
github.com/spf13/cobra.(*Command).ExecuteC(0xc00229cf00)
        github.com/spf13/cobra@v1.7.0/command.go:1068 +0x3bd
github.com/spf13/cobra.(*Command).Execute(0xc0022dd860?)
        github.com/spf13/cobra@v1.7.0/command.go:992 +0x19
main.runInteractive({{0xc0022dd860, 0xc0022ddad0, 0xc0022dda10, 0xc0022dd5f0, 0xc0022ddb00}, {{0x663630d, 0x7}, {0x0, 0x0}, {0x713fdf8, ...}}, ...})
        github.com/signalfx/splunk-otel-collector/cmd/otelcol/main.go:100 +0x5d
main.run({{0xc0022dd860, 0xc0022ddad0, 0xc0022dda10, 0xc0022dd5f0, 0xc0022ddb00}, {{0x663630d, 0x7}, {0x0, 0x0}, {0x713fdf8, ...}}, ...})
        github.com/signalfx/splunk-otel-collector/cmd/otelcol/main_windows.go:33 +0x58
main.main()
        github.com/signalfx/splunk-otel-collector/cmd/otelcol/main.go:93 +0xcba
```
**Link to tracking Issue:**
N/A

**Testing:**
Local test runs.

**Documentation:**
N/A

.chloggen/fix-panic-on-load_scraper_windows-shutdown.yaml

@@ -0,0 +1,27 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: receiver/hostmetrics
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Fix panic on load_scraper_windows shutdown
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [28678]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: []

receiver/hostmetricsreceiver/internal/scraper/loadscraper/load_scraper_windows.go

@@ -136,12 +136,18 @@ func stopSampling(_ context.Context) error {
 	startupLock.Lock()
 	defer startupLock.Unlock()
 
+	if scraperCount == 0 {
+		// no load scraper is running nothing to do
+		return nil
+	}
+
 	// only stop sampling if all load scrapers have been closed
 	scraperCount--
 	if scraperCount > 0 {
 		return nil
 	}
 
+	// no more load scrapers are running, stop the sampler
 	close(samplerInstance.done)
 	return nil
 }

receiver/hostmetricsreceiver/internal/scraper/loadscraper/load_scraper_windows_test.go

@@ -18,6 +18,13 @@ import (
 	"github.com/open-telemetry/opentelemetry-collector-contrib/receiver/hostmetricsreceiver/internal/perfcounters"
 )
 
+func TestStopSamplingWithoutStart(t *testing.T) {
+	// When the collector fails to start it is possible that stopSampling is called
+	// before startSampling. This test ensures that stopSampling does not panic in
+	// this scenario.
+	require.NoError(t, stopSampling(context.Background()))
+}
+
 func TestStartSampling(t *testing.T) {
 	t.Skip(t, "Test is causing race conditions, see https://github.com/open-telemetry/opentelemetry-collector-contrib/issues/10143.")
 	// override sampling frequency to 2ms