Do not attempt to transmit if not active

If AppSignal is not active, it's likely that there is no valid
configuration. Do not attempt to transmit heartbeats if AppSignal
is not active. Log a debug message about it.

lib/appsignal/heartbeat.rb

@@ -37,10 +37,15 @@ def event(kind)
     end
 
     def transmit_event(kind)
+      unless Appsignal.active?
+        Appsignal.internal_logger.debug("AppSignal not active, not transmitting heartbeat event")
+        return
+      end
+
       response = self.class.transmitter.transmit(event(kind))
 
       if response.code.to_i >= 200 && response.code.to_i < 300
-        Appsignal.internal_logger.trace("Transmitted heartbeat `#{name}` #{kind} event")
+        Appsignal.internal_logger.trace("Transmitted heartbeat `#{name}` (#{id}) #{kind} event")
       else
         Appsignal.internal_logger.error(
           "Failed to transmit heartbeat event: status code was #{response.code}"

spec/lib/appsignal/heartbeat_spec.rb

@@ -4,15 +4,26 @@
   let(:transmitter) { Appsignal::Transmitter.new("http://heartbeats/", config) }
 
   before(:each) do
+    allow(Appsignal).to receive(:active?).and_return(true)
     config.logger = Logger.new(StringIO.new)
     allow(Appsignal::Heartbeat).to receive(:transmitter).and_return(transmitter)
   end
 
+  describe "when Appsignal is not active" do
+    it "should not transmit any events" do
+      allow(Appsignal).to receive(:active?).and_return(false)
+      expect(transmitter).not_to receive(:transmit)
+
+      heartbeat.start
+      heartbeat.finish
+    end
+  end
+
   describe "#start" do
     it "should send a heartbeat start" do
       expect(transmitter).to receive(:transmit).with(hash_including(
         :name => "heartbeat-name",
-        :kind => "Start"
+        :kind => "start"
       )).and_return(nil)
 
       heartbeat.start
@@ -23,7 +34,7 @@
     it "should send a heartbeat finish" do
       expect(transmitter).to receive(:transmit).with(hash_including(
         :name => "heartbeat-name",
-        :kind => "Finish"
+        :kind => "finish"
       )).and_return(nil)
 
       heartbeat.finish
@@ -34,12 +45,12 @@
     describe "when a block is given" do
       it "should send a heartbeat start and finish and return the block output" do
         expect(transmitter).to receive(:transmit).with(hash_including(
-          :kind => "Start",
+          :kind => "start",
           :name => "heartbeat-with-block"
         )).and_return(nil)
 
         expect(transmitter).to receive(:transmit).with(hash_including(
-          :kind => "Finish",
+          :kind => "finish",
           :name => "heartbeat-with-block"
         )).and_return(nil)
 
@@ -67,7 +78,7 @@
     describe "when no block is given" do
       it "should only send a heartbeat finish event" do
         expect(transmitter).to receive(:transmit).with(hash_including(
-          :kind => "Finish",
+          :kind => "finish",
           :name => "heartbeat-without-block"
         )).and_return(nil)