realm_export: Improve estimate of data export size.

As suggested by the new comments, the cost for a Zulip data export
scales with messages actually included in the export, so an
organizations with 1M private messages but only 50K public stream
messages should not be modeled the same as one with 1M public stream
messages for the purpose of the limits here.

Also improve the comments and variable names more generally.

zerver/tests/test_realm_export.py

@@ -205,9 +205,9 @@ class RealmExportTest(ZulipTestCase):
         realm_count = RealmCount.objects.create(
             realm_id=admin.realm.id,
             end_time=timezone_now(),
-            subgroup=1,
             value=0,
-            property="messages_sent:client:day",
+            property="messages_sent:message_type:day",
+            subgroup="public_stream",
         )
 
         # Space limit is set as 10 GiB

zerver/views/realm_export.py

@@ -24,10 +24,13 @@ def export_realm(request: HttpRequest, user: UserProfile) -> HttpResponse:
     event_time = timezone_now()
     realm = user.realm
     EXPORT_LIMIT = 5
-    # Conservative limit on the size of message history in
-    # organizations being exported; this exists to protect Zulip
-    # against a possible unmonitored accidental DoS caused by trying
-    # to export an organization with huge history.
+
+    # Exporting organizations with a huge amount of history can
+    # potentially consume a lot of disk or otherwise have accidental
+    # DoS risk; for that reason, we require large exports to be done
+    # manually on the command line.
+    #
+    # It's very possible that higher limits would be completely safe.
     MAX_MESSAGE_HISTORY = 250000
     MAX_UPLOAD_QUOTA = 10 * 1024 * 1024 * 1024
 
@@ -40,14 +43,22 @@ def export_realm(request: HttpRequest, user: UserProfile) -> HttpResponse:
     if len(limit_check) >= EXPORT_LIMIT:
         raise JsonableError(_("Exceeded rate limit."))
 
-    total_messages = sum(
+    # The RealmCount analytics table lets us efficiently get an
+    # estimate for the number of public stream messages in an
+    # organization. It won't match the actual number of messages in
+    # the export, because this measures the number of messages that
+    # went to a public stream at the time they were sent. Thus,
+    # messages that were deleted or moved between streams will be
+    # treated differently for this check vs. in the export code.
+    exportable_messages_estimate = sum(
         realm_count.value
         for realm_count in RealmCount.objects.filter(
-            realm=user.realm, property="messages_sent:client:day"
+            realm=realm, property="messages_sent:message_type:day", subgroup="public_stream"
         )
     )
+
     if (
-        total_messages > MAX_MESSAGE_HISTORY
+        exportable_messages_estimate > MAX_MESSAGE_HISTORY
         or user.realm.currently_used_upload_space_bytes() > MAX_UPLOAD_QUOTA
     ):
         raise JsonableError(