keep track of missing live events context

why:
* to see if this is a problem, since events sent out without a context
will pose a problem to live events consumers

refs INTEROP-7187
flag=none

test plan:
* follow the instructions in doc/live_events.md to set up live events
and print them to stdout logs
* follow the instructions in doc/lti_manual/11_testing.md under
`Stubbing Statsd Calls` to monkey-patch Statsd for easy metric viewing
* (a good place to put that is in the ApplicationController)
* comment out the call to `LiveEvents.set_context` in the
ApplicationController
* trigger any live event - navigating through course pages should be
enough, though creating an assignment is always a good one
* for every event triggered there should be a Statsd call for
`missing_context`
* add back the call to `set_context`
* trigger more live events
* the Statsd calls should not continue

Change-Id: Ib87952e208885ef15b00561731769f808cdcd066
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/284053
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Tucker Mcknight <tmcknight@instructure.com>
Reviewed-by: Evan Battaglia <ebattaglia@instructure.com>
QA-Review: Evan Battaglia <ebattaglia@instructure.com>
Product-Review: Xander Moffatt <xmoffatt@instructure.com>

doc/lti_manual/11_testing.md

@@ -53,4 +53,31 @@ For testing new features that may not have been added to the test tool, or for m
 5. Include a request body, which could look like something like this:
   - [Line Item API docs](https://canvas.instructure.com/doc/api/line_items.html)
   - [Score API docs](https://canvas.instructure.com/doc/api/score.html)
-  - [Result API docs](https://canvas.instructure.com/doc/api/result.html)
+  - [Result API docs](https://canvas.instructure.com/doc/api/result.html)
+
+## Miscellaneous
+
+### Stubbing Statsd Calls
+
+This isn't exactly LTI-specific, but can come in handy. There isn't a great way to get
+visibility into the calls made to `InstStatsd::Statsd`, which is the way to get metrics
+sent to Datadog. It's possible to monkey-patch that class and stub the methods you care
+about and just log them to stdout. If you're working entirely in the Rails console, you
+can paste this patch directly into the console. If you're working with Canvas requests
+at all, you can paste this patch into any Ruby file in Canvas and save it (I prefer the
+one I'm currently working in, for consistency).
+
+Replace `increment` with any Statsd method you need, or add more methods if needed.
+
+```
+class << InstStatsd::Statsd
+  def increment(*args)
+    puts "DEBUG STATSD increment: #{args.to_json}"
+  end
+end
+```
+
+Then, look in the logs for your method calls. If you are working entirely in the Rails
+console, they will appear there. If you are working with Canvas requests, tail the web
+container's logs with `docker-compose logs -f --tail=100 web` and then search for
+`DEBUG STATSD`.

gems/live_events/lib/live_events.rb

@@ -108,6 +108,9 @@ module LiveEvents
       if context.is_a?(Proc)
         context = Thread.current[:live_events_ctx] = context.call
       end
+      if context.blank?
+        LiveEvents&.statsd&.increment("live_events.missing_context")
+      end
       context
     end