Record exception class name

If an exception happens during instrumentation with ActiveSupport the
payload will have a key :exception.

Report the exception class name as tag. Will make it possible to measure
"potential" problems. Potential as we don't know if the exception will
be handled higher up in the call chain.

CHANGELOG.md

@@ -14,6 +14,7 @@ For the full commit log, [see here](https://github.com/influxdata/influxdb-rails
 - Implement `perform_start.active_job` subscriber (https://guides.rubyonrails.org/active_support_instrumentation.html#perform-start-active-job)
 - Implement `perform.active_job` subscriber (https://guides.rubyonrails.org/active_support_instrumentation.html#perform-active-job)
 - Implement block instrumentation `InfluxDB::Rails.instrument do; 1 + 1; end`
+- Record unhandled exceptions as tags for process_action.action_controller
 
 ## v1.0.0, released 2019-10-23
 The Final release, no code changes.

README.md

@@ -71,10 +71,15 @@ Reported tags:
   method:      "PostsController#index",
   http_method: "GET",
   format:      "html",
-  status:      "200"
+  status:      ["200", ""]
+  exception:   ["", "ArgumentError"]
 }
 ```
 
+*Note*: If an exception happens during that particular instrumentation the
+`status` will be blank and the tag `exception` will contain the name of the
+exception class.
+
 ### Action View
 
 Reported ActiveSupport instrumentation hooks:

lib/influxdb/rails/middleware/request_subscriber.rb

@@ -19,6 +19,7 @@ def tags(payload)
             status:      payload[:status],
             format:      payload[:format],
             http_method: payload[:method],
+            exception:   payload[:exception]&.first,
           }
         end
 

spec/requests/action_controller_metrics_spec.rb

@@ -74,8 +74,9 @@
 
     expect_metric(
       tags: a_hash_including(
-        method: "ExceptionsController#index",
-        hook:   "process_action"
+        method:    "ExceptionsController#index",
+        hook:      "process_action",
+        exception: "ZeroDivisionError"
       )
     )
   end