Basic labels support

Adds basic labels support to the registry code (V2), and the API. Note that this does not yet add any UI related support.

util/label_validator.py

@@ -0,0 +1,20 @@
+class LabelValidator(object):
+  """ Helper class for validating that labels meet prefix requirements. """
+  def __init__(self, app):
+    self.app = app
+
+    overridden_prefixes = app.config.get('LABEL_KEY_RESERVED_PREFIXES', [])
+    for prefix in overridden_prefixes:
+      if not prefix.endswith('.'):
+        raise Exception('Prefix "%s" in LABEL_KEY_RESERVED_PREFIXES must end in a dot', prefix)
+
+    default_prefixes = app.config.get('DEFAULT_LABEL_KEY_RESERVED_PREFIXES', [])
+    self.reserved_prefixed_set = set(default_prefixes + overridden_prefixes)
+
+  def has_reserved_prefix(self, label_key):
+    """ Validates that the provided label key does not match any reserved prefixes. """
+    for prefix in self.reserved_prefixed_set:
+      if label_key.startswith(prefix):
+        return True
+
+    return False