Add active worker count to buildmanager logs.

buildman/manager/basemanager.py

@@ -47,3 +47,9 @@ class BaseManager(object):
         one of: incomplete, error, complete. If incomplete, the job should be requeued.
     """
     raise NotImplementedError
+
+  def num_workers(self):
+    """ Returns the number of active build workers currently registered. This includes those
+        that are currently busy and awaiting more work.
+    """
+    raise NotImplementedError

buildman/manager/enterprise.py

@@ -70,3 +70,5 @@ class EnterpriseManager(BaseManager):
   def build_component_disposed(self, build_component, timed_out):
     self.build_components.remove(build_component)
 
+  def num_workers(self):
+    return len(self.build_components)