Unmarshaling with call of viper.Get() for each struct field

[arxeiss]

We use a config file, ENV vars (automatic, without binding). Then we are loading credentials from Google Secret Manager, which is saved with viper.Set(). Then we want to unmarshal it into a structure, which is passed further.
But the problem with Unmarshal is, it just gets the top-level key, and does not merge it.

Example:
I have this in code viper.SetEnvPrefix("JKL"); viper.AutomaticEnv() and also env variable JKL_DB_HOST=localhost
and this is output from viper.Debug()

Override:
map[string]interface {}{"db":map[string]interface {}{"password":"pass", "username":"name"}}
Config:
map[string]interface {}{"db":map[string]interface {}{"timeout":15}}

When I do

viper.Get("db.timeout") // return 15
viper.Get("db.host") // return localhost
viper.Get("db.password") // return pass

// However

type DbConfig struct {
  Host string
  Username string
  Password string
  Timeout int
} 
dbcfg := &DbConfig{}
viper.UnmarshalKey("db", &dbcfg)
// dbcfg will have set only password and username

The reason is, that viper.Get("db") is returning the first match, which is in override, and do not merge with the rest.

Is there some way, which would just go through all fields in a struct and for each would call some callback, where I can put viper.Get()? Maybe some another library, because mapstruct obviously does not support it.