code: @Nullable
      public static EndpointGroup get(String groupName) {
          groupName = normalizeGroupName(groupName);
          final EndpointSelector endpointSelector = serverGroups.get(groupName);
          if (endpointSelector == null) {
              return null;
          }
          return endpointSelector.group();
      }
docs: Get the {@link EndpointGroup} for the specified case-insensitive {@code groupName}.
  
  @return the {@link EndpointSelector}, or {@code null} if {@code groupName} has not been registered yet.