Refactor server config GetConfig to use viper Unmarshal

[SpicyLemon]

Summary

Refactor the server/config.GetConfig function to use v.Unmarshal to load the config from viper instead of hard-coding each field manually.

Problem Definition

The current structure of the GetConfig function creates a common failure point for config related changes. The entire function could be simplified down to a v.Unmarshal call which would remove it as a failure point.

Proposal

Refactor GetConfig to this:

// GetConfig returns a fully parsed Config object.
func GetConfig(v *viper.Viper) (Config, error) {
	conf := DefaultConfig()
	if err := v.Unmarshal(conf); err != nil {
		return nil, fmt.Errorf("error extracting app config: %w", err)
	}
	return conf, nil
}

The special handling of telemetry.global-labels isn't needed; viper will unmarshal it just fine. The only different thing that handling does is silently ignore entries that have more or less than 2 entries. If that behavior is still desirable, I'd suggest adding it as some processing after using v.Unmarshal.

[tac0turtle]

this sounds good me,