[receiver/awscontainerinsight] Add HOST_PROC env var support (open-telemetry#37694)

#### Description
Add support to HOST_PROC env var support. At this point I'm not sure if
the default should be changed or not, but, with the env var one can at
least experiment with the receiver on different images.

#### Link to tracking issue
Fixes open-telemetry#35862

#### Testing
Added respective unit test.

#### Documentation
Changelog

Author: pjanotti

.chloggen/support-host-proc-env-var-on-awscontainerinsightreceiver.yaml

@@ -0,0 +1,11 @@
+change_type: enhancement
+
+component: awscontainerinsightreceiver
+
+note: Add support for HOST_PROC environment variable in AWS Container Insight Receiver.
+
+issues: [35862]
+
+subtext:
+
+change_logs: [user]

receiver/awscontainerinsightreceiver/internal/host/nodeCapacity.go

@@ -43,10 +43,15 @@ func newNodeCapacity(logger *zap.Logger, options ...nodeCapacityOption) (nodeCap
 		opt(nc)
 	}
 
-	if _, err := nc.osLstat(hostProc); os.IsNotExist(err) {
+	actualHostProc, ok := os.LookupEnv(string(common.HostProcEnvKey))
+	if !ok {
+		actualHostProc = hostProc
+	}
+
+	if _, err := nc.osLstat(actualHostProc); os.IsNotExist(err) {
 		return nil, err
 	}
-	envMap := common.EnvMap{common.HostProcEnvKey: hostProc}
+	envMap := common.EnvMap{common.HostProcEnvKey: actualHostProc}
 	ctx := context.WithValue(context.Background(), common.EnvKey, envMap)
 
 	nc.parseCPU(ctx)

receiver/awscontainerinsightreceiver/internal/host/nodeCapacity_test.go

@@ -9,6 +9,7 @@ import (
 	"os"
 	"testing"
 
+	"github.com/shirou/gopsutil/v4/common"
 	"github.com/shirou/gopsutil/v4/cpu"
 	"github.com/shirou/gopsutil/v4/mem"
 	"github.com/stretchr/testify/assert"
@@ -71,3 +72,19 @@ func TestNodeCapacity(t *testing.T) {
 	assert.Equal(t, int64(1024), nc.getMemoryCapacity())
 	assert.Equal(t, int64(2), nc.getNumCores())
 }
+
+func TestNodeCapacity_ReadsHostProcEnvVar(t *testing.T) {
+	const customHostProc = "/custom/host/proc"
+	t.Setenv(string(common.HostProcEnvKey), customHostProc)
+
+	lstatOption := func(nc *nodeCapacity) {
+		nc.osLstat = func(name string) (os.FileInfo, error) {
+			assert.Equal(t, customHostProc, name)
+			return nil, nil
+		}
+	}
+
+	nc, err := newNodeCapacity(zap.NewNop(), lstatOption)
+	assert.NoError(t, err)
+	assert.NotNil(t, nc)
+}