[core] Use constants for manual.keep and manual.drop (#919)

Author: brettlangdon

ddtrace/constants.py

@@ -5,3 +5,6 @@
 ORIGIN_KEY = '_dd.origin'
 
 NUMERIC_TAGS = (ANALYTICS_SAMPLE_RATE_KEY, )
+
+MANUAL_DROP_KEY = 'manual.drop'
+MANUAL_KEEP_KEY = 'manual.keep'

ddtrace/span.py

@@ -5,7 +5,7 @@
 import traceback
 
 from .compat import StringIO, stringify, iteritems, numeric_types
-from .constants import NUMERIC_TAGS
+from .constants import NUMERIC_TAGS, MANUAL_DROP_KEY, MANUAL_KEEP_KEY
 from .ext import errors, priority
 from .internal.logger import get_logger
 
@@ -138,10 +138,10 @@ def set_tag(self, key, value=None):
                 log.debug('error setting numeric metric {}:{}'.format(key, value))
 
             return
-        elif key == 'manual.keep':
+        elif key == MANUAL_KEEP_KEY:
             self.context.sampling_priority = priority.USER_KEEP
             return
-        elif key == 'manual.drop':
+        elif key == MANUAL_DROP_KEY:
             self.context.sampling_priority = priority.USER_REJECT
             return