added timer to trigger send if no incoming transactions trigger it

This should alleviate a point of confusion when users expected that we
send data every x seconds. Instead, we only sent data every x seconds
if there were incoming transactions. If no transactions were happening,
no data was sent (including existing transactions on the queue).

This PR aligns the behaviour with user expectations. The trade-offs are:

 * slightly more resource consumption due to the timer thread
 * collection can either happen on the main thread, or on the timer
   thread, depending on whether the timer or an incoming transaction
   triggers collection.

elasticapm/base.py

@@ -16,6 +16,7 @@
 import os
 import platform
 import socket
+import threading
 import sys
 import time
 import warnings
@@ -112,6 +113,8 @@ def __init__(self, config=None, **defaults):
             self.config.disable_send = True
             return
 
+        self._send_timer = None
+
         self._transport_class = import_string(self.config.transport_class)
         self._transports = {}
 
@@ -485,6 +488,8 @@ def handle_transport_fail(self, exception=None, **kwargs):
         self.state.set_fail()
 
     def _traces_collect(self):
+        if self._send_timer and self._send_timer.is_alive() and not self._send_timer == threading.current_thread():
+            self._send_timer.cancel()
         transactions = self.instrumentation_store.get_all()
         if not transactions:
             return
@@ -495,6 +500,8 @@ def _traces_collect(self):
         api_path = defaults.TRANSACTIONS_API_PATH
 
         self.send(server=self.config.server + api_path, **data)
+        self._send_timer = threading.Timer(self.config.traces_send_frequency, self._traces_collect)
+        self._send_timer.start()
 
     def get_app_info(self):
         language_version = platform.python_version()

elasticapm/transport/http_urllib3.py

@@ -1,5 +1,6 @@
 # -*- coding: utf-8 -*-
 import os
+import logging
 
 import certifi
 import urllib3
@@ -11,6 +12,9 @@
 from elasticapm.utils import compat
 
 
+logger = logging.getLogger(__name__)
+
+
 class Urllib3Transport(HTTPTransport):
 
     scheme = ['http', 'https']
@@ -46,6 +50,7 @@ def send(self, data, headers, timeout=None):
                 response = self.http.urlopen(
                     'POST', url, body=data, headers=headers, timeout=timeout, preload_content=False
                 )
+                logger.info('Sent request, url=%s size=%.2fkb status=%s', url, len(data) / 1024.0, response.status)
             except Exception as e:
                 print_trace = True
                 if isinstance(e, MaxRetryError) and isinstance(e.reason, TimeoutError):