Fix setting of "action" for Grape transactions

Merely setting the "action" tag will only result in the transaction
itself containing a value for this tag. To ensure other metrics also
contain this tag we must set the action using Transaction#action=
instead.

lib/api/internal.rb

@@ -23,7 +23,7 @@ module API
       end
       post "/allowed" do
-        Gitlab::Metrics.tag_transaction('action', 'Grape#/internal/allowed')
+        Gitlab::Metrics.action = 'Grape#/internal/allowed'
         status 200

lib/gitlab/metrics.rb

@@ -115,6 +115,15 @@ module Gitlab
       trans.add_tag(name, value) if trans
     end
+    # Sets the action of the current transaction (if any)
+    #
+    # action - The name of the action.
+    def self.action=(action)
+      trans = current_transaction
+
+      trans.action = action if trans
+    end
+
     # When enabled this should be set before being used as the usual pattern
     # "@foo ||= bar" is _not_ thread-safe.
     if enabled?

spec/lib/gitlab/metrics_spec.rb

@@ -123,4 +123,28 @@ describe Gitlab::Metrics do
       end
     end
   end
+
+  describe '.action=' do
+    context 'without a transaction' do
+      it 'does nothing' do
+        expect_any_instance_of(Gitlab::Metrics::Transaction).
+          not_to receive(:action=)
+
+        Gitlab::Metrics.action = 'foo'
+      end
+    end
+
+    context 'with a transaction' do
+      it 'sets the action of a transaction' do
+        trans = Gitlab::Metrics::Transaction.new
+
+        expect(Gitlab::Metrics).to receive(:current_transaction).
+          and_return(trans)
+
+        expect(trans).to receive(:action=).with('foo')
+
+        Gitlab::Metrics.action = 'foo'
+      end
+    end
+  end
 end