Make sure that no two given configs apply to the same interceptor

This is a TODO expressed here:

Lines 225 to 233 in d29986c

    
           var cfg InterceptorConfig 
        
           for _, c := range cfgs { 
        
           	if c.Match(it) { 
        
           		// TODO: there should be a validation check that there is at 
        
           		// most one config per interceptor. 
        
           		cfg = c 
        
           		break 
        
           	} 
        
           }

We should either panic there, or require configs to be somehow mergeable.

	var cfg InterceptorConfig
	for _, c := range cfgs {
	if c.Match(it) {
	// TODO: there should be a validation check that there is at
	// most one config per interceptor.
	cfg = c
	break
	}
	}