Add support for library instrumentation

[losalex]

This feature provides an ability to log extra entry with diagnostics structure which contains logging library information. Such entry is logged only once when first entry is written by a process using logging library. In the future, the diagnostic record could be extended to include more details about libraries used, runtime environment, etc. Below is example of how the logging record with diagnostic info looks:

{
 "jsonPayload": {
   "logging.googleapis.com/diagnostic": {
     "instrumentation_source": [
       {
         "name": "nodejs",
         "version": "10.0.1"
       },
       {
         "name": "nodejs-bunyan",
         "version": "4.0.0"
       }
     ]
   }
 },
 "insertId": "..........BB_7geD6W7jQV1oz_bQ_ee",
 "severity": "INFO",
 "logName": "projects/startup/logs/hello-log-bunyan",
}