make logging keys fluentD compatible

[ashmeenkaur]

Description

This PR changes GCSFuse logging keys to be compatible with fluentD based on the following references:

1. https://cloud.google.com/logging/docs/agent/logging/configuration#timestamp-processing
2. https://cloud.google.com/logging/docs/agent/logging/configuration#special-fields

Changes summary:

1. JSON log structure time changed
   "time":{"timestampSeconds":1694164762,"timestampNanos":1694164762084862036}
   to
   "timestamp":{"seconds":1704697907,"nanos":553918512}

2. JSON log structure message changed from
   "msg":"<log message>"
   to
   "message":"<log message>"

Sample log:

{"timestamp":{"seconds":1704703429,"nanos":670828386},"severity":"TRACE","message":"fuse_debug: Op 0x00000144        connection.go:415] <- ReleaseDirHandle (PID 0)"}

Link to the issue in case of a bug fix.

NA

Testing details

1. Manual - Manually verified the log format is working with google fluentD
2. Unit tests - Changed unit tests to match regular expression of log.
3. Integration tests - NA