fix(inputs.gnmi): Refactor alias handling

Rather than abritrarily take the substring of key, ensure that the key
contains the alias in the first place, then do the removal. Otherwise,
always take the base path.

fixes: #14530

plugins/inputs/gnmi/handler.go

@@ -241,12 +241,9 @@ func (h *handler) handleSubscribeResponseUpdate(acc telegraf.Accumulator, respon
 				key = parts[1]
 			}
 		} else {
-			if len(aliasPath) < len(key) && len(aliasPath) != 0 {
-				// This may not be an exact prefix, due to naming style
-				// conversion on the key.
+			if len(aliasPath) != 0 && strings.Contains(key, aliasPath) && len(aliasPath) < len(key) {
 				key = key[len(aliasPath)+1:]
-			} else if len(aliasPath) >= len(key) {
-				// Otherwise use the last path element as the field key.
+			} else {
 				key = path.Base(key)
 			}
 		}