[NO-TICKET] Fix rubocop lint

**What does this PR do?**

This PR moves the `start_worker` and `stop_worker` to the `private`
section of `writer.rb`.

It does this instead of using
`private :start_worker, :stop_worker` to make rubocop happy.

**Motivation:**

Unbreak rubocop in master.

**Additional Notes:**

I thought we had pinned down the version of rubocop in CI, but it
appears we have a gap. I've not investigated further.

**How to test the change?**

Validate that the lint check in CI is back to green.

lib/datadog/tracing/writer.rb

@@ -68,21 +68,6 @@ def start
         end
       end
 
-      # spawns a worker for spans; they share the same transport which is thread-safe
-      # @!visibility private
-      def start_worker
-        @trace_handler = ->(items, transport) { send_spans(items, transport) }
-        @worker = Workers::AsyncTransport.new(
-          transport: @transport,
-          buffer_size: @buff_size,
-          on_trace: @trace_handler,
-          interval: @flush_interval,
-          shutdown_timeout: @shutdown_timeout
-        )
-
-        @worker.start
-      end
-
       # Gracefully shuts down this writer.
       #
       # Once stopped methods calls won't fail, but
@@ -93,19 +78,6 @@ def stop
         @mutex_after_fork.synchronize { stop_worker }
       end
 
-      def stop_worker
-        @stopped = true
-
-        return if @worker.nil?
-
-        @worker.stop
-        @worker = nil
-
-        true
-      end
-
-      private :start_worker, :stop_worker
-
       # flush spans to the trace-agent, handles spans only
       # @!visibility private
       def send_spans(traces, transport)
@@ -179,6 +151,32 @@ def initialize
 
       private
 
+      # spawns a worker for spans; they share the same transport which is thread-safe
+      # @!visibility private
+      def start_worker
+        @trace_handler = ->(items, transport) { send_spans(items, transport) }
+        @worker = Workers::AsyncTransport.new(
+          transport: @transport,
+          buffer_size: @buff_size,
+          on_trace: @trace_handler,
+          interval: @flush_interval,
+          shutdown_timeout: @shutdown_timeout
+        )
+
+        @worker.start
+      end
+
+      def stop_worker
+        @stopped = true
+
+        return if @worker.nil?
+
+        @worker.stop
+        @worker = nil
+
+        true
+      end
+
       def reset_stats!
         @traces_flushed = 0
         @transport.stats.reset!