[receiver/hostmetrics] respect env var use in filesystem scraper (#35045

)

**Description:** 
In filesystem scraper, do not prefix partitions when using the
environment variable HOST_PROC_MOUNTINFO

**Link to tracking Issue:**
Fixes #35043

.chloggen/fix_envvar_filesystem.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: hostmetricsreceiver
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: In filesystem scraper, do not prefix partitions when using the environment variable HOST_PROC_MOUNTINFO
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [35043]
+
+# (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/filesystemscraper/filesystem_scraper.go

@@ -6,6 +6,7 @@ package filesystemscraper // import "github.com/open-telemetry/opentelemetry-col
 import (
 	"context"
 	"fmt"
+	"os"
 	"path/filepath"
 	"strings"
 	"time"
@@ -162,5 +163,8 @@ func (f *fsFilter) includeMountPoint(mountPoint string) bool {
 
 // translateMountsRootPath translates a mountpoint from the host perspective to the chrooted perspective.
 func translateMountpoint(rootPath, mountpoint string) string {
+	if mountInfo := os.Getenv("HOST_PROC_MOUNTINFO"); mountInfo != "" {
+		return mountpoint
+	}
 	return filepath.Join(rootPath, mountpoint)
 }

receiver/hostmetricsreceiver/internal/scraper/filesystemscraper/filesystem_scraper_test.go

@@ -30,6 +30,7 @@ func TestScrape(t *testing.T) {
 		name                     string
 		config                   Config
 		rootPath                 string
+		osEnv                    map[string]string
 		bootTimeFunc             func(context.Context) (uint64, error)
 		partitionsFunc           func(context.Context, bool) ([]disk.PartitionStat, error)
 		usageFunc                func(context.Context, string) (*disk.UsageStat, error)
@@ -195,6 +196,43 @@ func TestScrape(t *testing.T) {
 				},
 			},
 		},
+		{
+			name: "RootPath at /hostfs but HOST_PROC_MOUNTINFO is set",
+			osEnv: map[string]string{
+				"HOST_PROC_MOUNTINFO": "/proc/1/self",
+			},
+			config: Config{
+				MetricsBuilderConfig: metadata.DefaultMetricsBuilderConfig(),
+			},
+			rootPath: filepath.Join("/", "hostfs"),
+			usageFunc: func(_ context.Context, s string) (*disk.UsageStat, error) {
+				if s != "mount_point_a" {
+					return nil, errors.New("mountpoint translated according to RootPath")
+				}
+				return &disk.UsageStat{
+					Fstype: "fs_type_a",
+				}, nil
+			},
+			partitionsFunc: func(context.Context, bool) ([]disk.PartitionStat, error) {
+				return []disk.PartitionStat{
+					{
+						Device:     "device_a",
+						Mountpoint: "mount_point_a",
+						Fstype:     "fs_type_a",
+					},
+				}, nil
+			},
+			expectMetrics:            true,
+			expectedDeviceDataPoints: 1,
+			expectedDeviceAttributes: []map[string]pcommon.Value{
+				{
+					"device":     pcommon.NewValueStr("device_a"),
+					"mountpoint": pcommon.NewValueStr("mount_point_a"),
+					"type":       pcommon.NewValueStr("fs_type_a"),
+					"mode":       pcommon.NewValueStr("unknown"),
+				},
+			},
+		},
 		{
 			name: "Invalid Include Device Filter",
 			config: Config{
@@ -314,7 +352,9 @@ func TestScrape(t *testing.T) {
 	for _, test := range testCases {
 		test := test
 		t.Run(test.name, func(t *testing.T) {
-			t.Parallel()
+			for k, v := range test.osEnv {
+				t.Setenv(k, v)
+			}
 			test.config.SetRootPath(test.rootPath)
 			scraper, err := newFileSystemScraper(context.Background(), receivertest.NewNopSettings(), &test.config)
 			if test.newErrRegex != "" {