hostmetricsreceiver: use gopsutil cached boot time (open-telemetry#32126

)

**Description:**
This PR enables `gopsutil`'s cached boot time feature. The primary
purpose is to reduce the CPU usage of the `process` and `processes`
scrapers, which read the boot time vastly more times than necessary.

Also added the `hostmetrics.process.bootTimeCache` featuregate which is
enabled by default. Disabling it will return the `process` scraper to a
similar functionality of reading the boot time at the start of every
scrape (but still won't read it as much as it used to).

**Link to tracking Issue:** open-telemetry#28849

**Testing:** 
No tests were added. Ran the collector to ensure that everything still
functioned as expected with the new functionality.

.chloggen/hostmetrics-boot-time-cache.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: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: hostmetricsreceiver
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: The hostmetricsreceiver now caches the system boot time at receiver start and uses it for all subsequent calls. The featuregate `hostmetrics.process.bootTimeCache` can be disabled to restore previous behaviour.
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [28849]
+
+# (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: This change was made because it greatly reduces the CPU usage of the process and processes scrapers.
+
+# 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: [user]

receiver/hostmetricsreceiver/factory.go

@@ -8,6 +8,8 @@ import (
 	"fmt"
 	"os"
 
+	"github.com/shirou/gopsutil/v3/host"
+	"github.com/shirou/gopsutil/v3/process"
 	"go.opentelemetry.io/collector/component"
 	"go.opentelemetry.io/collector/consumer"
 	"go.opentelemetry.io/collector/receiver"
@@ -76,6 +78,9 @@ func createMetricsReceiver(
 		return nil, err
 	}
 
+	host.EnableBootTimeCache(true)
+	process.EnableBootTimeCache(true)
+
 	return scraperhelper.NewScraperControllerReceiver(
 		&oCfg.ControllerConfig,
 		set,

receiver/hostmetricsreceiver/internal/scraper/processscraper/factory.go

@@ -8,6 +8,7 @@ import (
 	"errors"
 	"runtime"
 
+	"go.opentelemetry.io/collector/featuregate"
 	"go.opentelemetry.io/collector/receiver"
 	"go.opentelemetry.io/collector/receiver/scraperhelper"
 
@@ -22,6 +23,17 @@ const (
 	TypeStr = "process"
 )
 
+var (
+	bootTimeCacheFeaturegateID = "hostmetrics.process.bootTimeCache"
+	bootTimeCacheFeaturegate   = featuregate.GlobalRegistry().MustRegister(
+		bootTimeCacheFeaturegateID,
+		featuregate.StageBeta,
+		featuregate.WithRegisterDescription("When enabled, all process scrapes will use the boot time value that is cached at the start of the process."),
+		featuregate.WithRegisterReferenceURL("https://github.com/open-telemetry/opentelemetry-collector-contrib/issues/28849"),
+		featuregate.WithRegisterFromVersion("v0.98.0"),
+	)
+)
+
 // Factory is the Factory for scraper.
 type Factory struct {
 }

receiver/hostmetricsreceiver/internal/scraper/processscraper/process_scraper.go

@@ -12,6 +12,7 @@ import (
 
 	"github.com/shirou/gopsutil/v3/common"
 	"github.com/shirou/gopsutil/v3/cpu"
+	"github.com/shirou/gopsutil/v3/host"
 	"github.com/shirou/gopsutil/v3/process"
 	"go.opentelemetry.io/collector/component"
 	"go.opentelemetry.io/collector/pdata/pcommon"
@@ -104,6 +105,19 @@ func (s *scraper) start(context.Context, component.Host) error {
 func (s *scraper) scrape(ctx context.Context) (pmetric.Metrics, error) {
 	var errs scrapererror.ScrapeErrors
 
+	// If the boot time cache featuregate is disabled, this will refresh the
+	// cached boot time value for use in the current scrape. This functionally
+	// replicates the previous functionality in all but the most extreme
+	// cases of boot time changing in the middle of a scrape.
+	if !bootTimeCacheFeaturegate.IsEnabled() {
+		host.EnableBootTimeCache(false)
+		_, err := host.BootTimeWithContext(ctx)
+		if err != nil {
+			errs.AddPartial(1, fmt.Errorf(`retrieving boot time failed with error "%w", using cached boot time`, err))
+		}
+		host.EnableBootTimeCache(true)
+	}
+
 	data, err := s.getProcessMetadata()
 	if err != nil {
 		var partialErr scrapererror.PartialScrapeError