Read the proc status file when PID is given for metrics purpose.

To measure the MaxRSS memory correctly, read the proc status file
under /proc/<pid>/status and extract the MaxRSS value from it. The
implementation is only available for Linux based distributions.

Bug: b/169453825
Test: go test
Change-Id: I32e3068fee7447f9ef5dfb5a8d8dcb6934e0af23

ui/metrics/proc/status_linux.go

@@ -0,0 +1,46 @@
+package proc
+
+import (
+	"io/ioutil"
+	"path/filepath"
+	"strconv"
+	"strings"
+
+	"android/soong/finder/fs"
+)
+
+// NewProcStatus returns an instance of the ProcStatus that contains memory
+// information of the process. The memory information is extracted from the
+// "/proc/<pid>/status" text file. This is only available for Linux
+// distribution that supports /proc.
+func NewProcStatus(pid int, fileSystem fs.FileSystem) (*ProcStatus, error) {
+	statusFname := filepath.Join("/proc", strconv.Itoa(pid), "status")
+	r, err := fileSystem.Open(statusFname)
+	if err != nil {
+		return &ProcStatus{}, err
+	}
+	defer r.Close()
+
+	data, err := ioutil.ReadAll(r)
+	if err != nil {
+		return &ProcStatus{}, err
+	}
+
+	s := &ProcStatus{
+		pid: pid,
+	}
+
+	for _, l := range strings.Split(string(data), "\n") {
+		// If the status file does not contain "key: values", just skip the line
+		// as the information we are looking for is not needed.
+		if !strings.Contains(l, ":") {
+			continue
+		}
+
+		// At this point, we're only considering entries that has key, single value pairs.
+		kv := strings.SplitN(l, ":", 2)
+		fillProcStatus(s, strings.TrimSpace(kv[0]), strings.TrimSpace(kv[1]))
+	}
+
+	return s, nil
+}