Add Logger#<< to ad a raw unformatted message to the buffer

If the current Buffer is bufering, the message will just be added. Else,
it will be flushed to the sink directly.

lib/rackstash/logger.rb

@@ -56,6 +56,21 @@ module Rackstash
       @buffer_stack = Concurrent::ThreadLocalVar.new
     end
 
+    # Add a message to the current buffer without any further formatting. If
+    # the current {Buffer} is bufering, the message will just be added. Else,
+    # it will be flushed to the {#sink} directly.
+    #
+    # @param msg [Object]
+    # @return [String] the passed `msg`
+    def <<(msg)
+      buffer.add_message Message.new(
+        msg,
+        time: Time.now.utc.freeze,
+      )
+      msg
+    end
+
+
     # Set the base log level as either one of the {SEVERITIES} or a
     # String/Symbol describing the level. When logging a message, it will only
     # be added if its log level is at or above the base level defined here

spec/rackstash/logger_spec.rb

@@ -348,6 +348,12 @@ describe Rackstash::Logger do
       logger.unknown { 'Unknown' }
       expect(messages.last).to include message: 'Unknown', severity: 5
     end
+
+    it 'can add a raw message with <<' do
+      logger << :raw_value
+      expect(messages.last).to include message: :raw_value
+      expect(messages.last).not_to include :formatter, :severity
+    end
   end
 
   describe '#with_buffer' do

spec/rackstash/message_spec.rb

@@ -174,10 +174,11 @@ describe Rackstash::Message do
     end
 
     it 'accepts non-string objects' do
-      exception = StandardError.new('An error')
-      message = Rackstash::Message.new(exception)
-
+      message = Rackstash::Message.new(StandardError.new('An error'))
       expect(message.to_s).to eql '#<StandardError: An error>'
+
+      message = Rackstash::Message.new(:symbol)
+      expect(message.to_s).to eql ':symbol'
     end
 
     it 'is aliased to to_str' do