getting indiv props instead of reading all props (#444)

Secured applications may not have access to all system properties on
a server. Making this change to prevent reading all the system properties
and just read the properties that are required for creating the
DefaultMonitorRegistry instance.

Fixes #443.

servo-aws/src/main/java/com/netflix/servo/tag/aws/AwsInjectableTag.java

@@ -89,7 +89,7 @@ public String tagString() {
 
   static String getAutoScaleGroup() {
     try {
-      String credFileProperty = System.getProperties().getProperty(
+      String credFileProperty = System.getProperty(
           AwsPropertyKeys.AWS_CREDENTIALS_FILE.getBundle());
       AWSCredentials credentials;
 

servo-core/src/main/java/com/netflix/servo/DefaultMonitorRegistry.java

@@ -61,7 +61,7 @@ public static MonitorRegistry getInstance() {
    * Creates a new instance based on system properties.
    */
   DefaultMonitorRegistry() {
-    this(System.getProperties());
+    this(loadProps());
   }
 
   /**
@@ -118,6 +118,24 @@ private static ObjectNameMapper getObjectNameMapper(Properties props) {
 
     return mapper;
   }
+
+  private static Properties loadProps() {
+    String registryClassProp = System.getProperty(REGISTRY_CLASS_PROP);
+    String registryNameProp = System.getProperty(REGISTRY_NAME_PROP);
+    String registryJmxNameProp = System.getProperty(REGISTRY_JMX_NAME_PROP);
+
+    Properties props = new Properties();
+    if (registryClassProp != null) {
+      props.setProperty(REGISTRY_CLASS_PROP, registryClassProp);
+    }
+    if (registryNameProp != null) {
+      props.setProperty(REGISTRY_NAME_PROP, registryNameProp);
+    }
+    if (registryJmxNameProp != null) {
+      props.setProperty(REGISTRY_JMX_NAME_PROP, registryJmxNameProp);
+    }
+    return props;
+  }
 
   /**
    * The set of registered Monitor objects.