[DI] Add debug logs when starting/stopping the CDP session (#5694)

Before the first probe is added, the Chrome DevTools Protocol (CDP)
sesion is started and after the last probe has been deleted/deactivated,
the CDP session is stopped.

When these events happen, log a debug message.

Author: watson

packages/dd-trace/src/debugger/devtools_client/breakpoints.js

@@ -175,11 +175,13 @@ async function updateBreakpoint (breakpoint, probe) {
 
 function start () {
   sessionStarted = true
+  log.debug('[debugger:devtools_client] Starting debugger')
   return session.post('Debugger.enable')
 }
 
 function stop () {
   sessionStarted = false
+  log.debug('[debugger:devtools_client] Stopping debugger')
   return session.post('Debugger.disable')
 }