[2225_statistics] rename _stoptimer() to stop(), change its interface, and update the description

It doesn't raise DataNotFound if the timer isn't started but it raises if the
timer is stopped, which isn't defined in the spec file.

src/lib/python/isc/statistics/counter.py

@@ -258,26 +258,33 @@ class _Counter():
         identifier = '/'.join(args)
         isc.cc.data.set(self._start_time, identifier, _start_timer())
 
-    def _stoptimer(self, identifier):
+    def stop(self, *args):
         """Sets duration time between corresponding time in
         self._start_time and current time into the value of the
         identifier. It deletes corresponding time in self._start_time
-        after setting is successfully done. If DataNotFoundError is
-        raised while invoking _stop_timer(), it stops setting and
-        ignores the exception."""
+        after setting is successfully done. In case of stopping the
+        timer which has never been started, it raises and does
+        nothing. But in case of stopping the time which isn't defined
+        in the spec file, it raises DataNotFoundError"""
+        identifier = '/'.join(args)
         try:
+            start_time = isc.cc.data.find(self._start_time,
+                                          identifier)
+        except isc.cc.data.DataNotFoundError:
+            # do not set the end time if the timer isn't started
+            pass
+        else:
+            # set the end time
             _stop_timer(
-                isc.cc.data.find(self._start_time, identifier),
+                start_time,
                 self._statistics_data,
                 self._statistics_spec,
                 identifier)
+            # delete the started timer
             del isc.cc.data.find(
                 self._start_time,
                 '/'.join(identifier.split('/')[0:-1]))\
                 [identifier.split('/')[-1]]
-        except isc.cc.data.DataNotFoundError:
-            # do not set end time if it's not started
-            pass
 
     def dump_statistics(self):
         """Calculates an entire server counts, and returns statistics