text: Title: [APM] Convert camelCase to snake_case for all files and folders
  Body: Kibana is using snake_case for all files and folders but APM has a mixed filenaming scheme. APM should use snake_case like the rest of Kibana.
  APM has disabled Kibana's filename check. This check should be re-enabled again.
  
  **Steps**
  1. Remove/uncomment the following lines to re-add case-checking of APM files:
  https://github.com/elastic/kibana/blob/9d42d670e1676a622d7ee7d4d564c4d7dc3973e8/src/dev/precommit_hook/casing_check_config.js#L60-L64
  
  2. Run `node scripts/check_file_casing.js` from kibana root
  
  3. Fix all the filename errors listed in the step above
  4. Run `node scripts/check_file_casing.js` again to verify all errors are fixed
  
  **Notes**
  If you're on a platform with a case-insensitive filesystem (macOS by default), you'll need to have
  
  ```
  [core]
  	ignorecase = false
  ```
  
  in .git/config.
  
  If you're _just_ changing the casing of a filename (from upper to lowercase) you may need to use `git mv -f` to rename the files and make sure github notices.
  
  **Tasks** 
  - [x] `x-pack/plugins/apm/public/**/*`
  - [x] `x-pack/plugins/apm/scripts/**/* `