Impossible to ingest timestamp from nested json

[tumd]

Describe the bug

Similar to #1013, I try to parse a timestamp field located under a nested key in a json structured log.
My particular logs are from MongoDB. Please see it's documentation for more example logs.

To Reproduce

Example log that should be possible to parse;

{"t":{"$date":"2020-05-01T15:16:17.180+00:00"},"s":"I", "c":"NETWORK", "id":12345, "ctx":"listener", "msg":"Listening on","attr":{"address":"127.0.0.1"}}

[PARSER]
  Name   mongodb-json
  Format json
  Time_Key t.$date
  Time_Format %Y-%m-%dT%H:%M:%S.%L%z

[SERVICE]
  Log_Level debug
  Parsers_File parsers.conf

[INPUT]
  Name tail
  Path mongos.log
  Parser mongodb-json
  Read_from_Head true
  Exit_On_Eof true

[OUTPUT]
  Name   stdout
  Match  *

The problem is; The field containing the timestamp is impossible to parse as such. I have been unable to find any working workaround to get this field parsed.

Please either add support to use Time_Key with nested fields, or suggest a reasonable & working workaround.

I have tried;

• to set the Parser Time_Key in various different variations to suit the nested key;
  • Time_Key t.$date
  • Time_Key t.\$date
  • Time_Key "t.$date"
  • Time_Key "t.\$date"
  • Time_Key t.$$date
  • ...
• to use the "Record Accessor" in the parser to reach the timestamp. Like so; Time_Key $t.["$date"].
• a whole bunch of different nest-filter variations, including lift $date from t without luck.
• to parse the t key as json with another parser-filter:

[PARSER]
 Name   json
 Format json

[PARSER]
 Name   mongodb-json
 Format json
 Time_Key $date
 Time_Format %Y-%m-%dT%H:%M:%S.%L%z

[INPUT]
  Name tail
  Path mongos.log
  Parser json
  Read_from_Head true
  Exit_On_Eof true

[FILTER]
  Name parser
  Parser mongodb-json
  Key_name t

Initially I figured that the key-name $date containing a $ was the issue, but it seem not to be.
A manually crafted log-entry where the t-key have been flatted out works great with bellow parser;

{"$date":"2020-05-01T15:16:17.180+00:00","s":"I", "c":"NETWORK", "id":12345, "ctx":"listener", "msg":"Listening on","attr":{"address":"127.0.0.1"}}

[PARSER]
  Name   mongodb-json
  Format json
  Time_Key $date
  Time_Format %Y-%m-%dT%H:%M:%S.%L%z

Expected behavior

That the nested t.$date may be possible to use as a timestamp field.

Screenshots

Your Environment

• Version used: v3.0.3
• Configuration: See above
• Environment name and version (e.g. Kubernetes? What version?): Nah
• Server type and version: GCP e2-medium
• Operating System and version: Debian GNU/Linux 11 (bullseye)
• Filters and plugins: Se above

Additional context