Move stats collection sample rate to config

This way, we can update the rate in one place.

r2/example.ini

@@ -202,6 +202,8 @@ error_reporters =
 statsd_addr =
 # how often to send them [0.0 - 1.0]
 statsd_sample_rate = 1.0
+# percentage of stats for sampling (0-100)
+stats_sample_rate = 1
 
 
 ############################################ MEDIA STORAGE

r2/r2/lib/template_helpers.py

@@ -151,6 +151,7 @@ def js_config(extra_config=None):
         # where do ajax requests go?
         "ajax_domain": get_domain(cname=c.authorized_cname, subreddit=False),
         "stats_domain": g.stats_domain or '',
+        "stats_sample_rate": g.stats_sample_rate or 0,
         "extension": c.extension,
         "https_endpoint": is_subdomain(request.host, g.domain) and g.https_endpoint,
         # does the client only want to communicate over HTTPS?

r2/r2/public/static/js/ui.js

@@ -86,11 +86,15 @@ r.ui.initLiveTimestamps = function() {
 }
 
 r.ui.initTimings = function() {
-  // sample at a rate of 1%
-  if (Math.random() > 0.01) { return }
+  // return if we're not configured for sending stats
+  if (!r.config.pageInfo.actionName || !r.config.stats_domain) {
+    return
+  }
 
-  if (!r.config.pageInfo.actionName) { return }
-  if (!r.config.stats_domain) { return }
+  // Sample based on the configuration sample rate
+  if (Math.random() > r.config.stats_sample_rate / 100) {
+    return
+  }
 
   var browserTimings = new r.NavigationTimings()