support linkerd (#547)

* support linkerd - initial commit

* renaming readEnvironmentVariable

tap/source/discoverer_util.go

@@ -0,0 +1,38 @@
+package source
+
+import (
+	"io/ioutil"
+	"regexp"
+	"strings"
+
+	"github.com/up9inc/mizu/shared/logger"
+)
+
+var numberRegex = regexp.MustCompile("[0-9]+")
+
+func getSingleValueFromEnvironmentVariableFile(filePath string, variableName string) (string, error) {
+	bytes, err := ioutil.ReadFile(filePath)
+
+	if err != nil {
+		logger.Log.Warningf("Error reading environment file %v - %v", filePath, err)
+		return "", err
+	}
+
+	envs := strings.Split(string(bytes), string([]byte{0}))
+
+	for _, env := range envs {
+		if !strings.Contains(env, "=") {
+			continue
+		}
+
+		parts := strings.Split(env, "=")
+		varName := parts[0]
+		value := parts[1]
+
+		if variableName == varName {
+			return value, nil
+		}
+	}
+
+	return "", nil
+}