Enable use of wal-g config file

[strowi]

Hi,

i am interested in exporting metrics from a wal-g instance that streams the arhive to S3 and uses a config-file similar to:

{
  "WALG_S3_PREFIX": "s3://",
  "AWS_ACCESS_KEY_ID": "",
  "AWS_SECRET_ACCESS_KEY": "",
  "AWS_ENDPOINT": "",
  "AWS_S3_FORCE_PATH_STYLE": "true",
  "WALG_S3_CA_CERT_FILE": "",
  "PGPASSWORD": "",
  "PGHOST": "",
  "PGPORT": 5432
}

And run as wal-g --config /x/y/z.json backup-list.

It would be very useful to be able to specify the same config to the exporters. Currently i am getting an error when the exporter runs wal-g backup-list...

Might be even worth using the wal-g config as input for the exporter..?