Bug 1898737 - Add telemetry for byte-range requests, and how often they're cacheable r=jesup,kershaw,necko-reviewers

Differential Revision: https://phabricator.services.mozilla.com/D212825

netwerk/protocol/http/metrics.yaml

@@ -56,6 +56,21 @@ netwerk:
     expires: never
 
 network:
+  byte_range_request:
+    type: labeled_counter
+    labels:
+      - cacheable
+      - not_cacheable
+    description: >
+      Counts of cacheable/non-cacheable byte-range requests
+    bugs:
+      - https://bugzilla.mozilla.org/show_bug.cgi?id=1898737
+    data_reviews:
+      - https://bugzilla.mozilla.org/show_bug.cgi?id=1898737
+    notification_emails:
+      - necko@mozilla.com
+    expires: never
+
   data_size_per_type:
     type: labeled_counter
     labels:

netwerk/protocol/http/nsHttpChannel.cpp

@@ -3749,7 +3749,17 @@ static bool IsSubRangeRequest(nsHttpRequestHead& aRequestHead) {
   if (NS_FAILED(aRequestHead.GetHeader(nsHttp::Range, byteRange))) {
     return false;
   }
-  return !byteRange.EqualsLiteral("bytes=0-");
+
+  if (byteRange.EqualsLiteral("bytes=0-")) {
+#ifndef ANDROID
+    glean::network::byte_range_request.Get("cacheable"_ns).Add(1);
+#endif
+    return false;
+  }
+#ifndef ANDROID
+  glean::network::byte_range_request.Get("not_cacheable"_ns).Add(1);
+#endif
+  return true;
 }
 
 nsresult nsHttpChannel::OpenCacheEntry(bool isHttps) {