code: @CheckForNull
      @Restricted(NoExternalUse.class)
      public String getMetadataUrlForDownloadable(String downloadable) {
          String siteUrl = getUrl();
          String updateSiteMetadataUrl = null;
          int baseUrlEnd = siteUrl.indexOf("update-center.json");
          if (baseUrlEnd != -1) {
              String siteBaseUrl = siteUrl.substring(0, baseUrlEnd);
              updateSiteMetadataUrl = siteBaseUrl + "updates/" + downloadable;
          } else {
              LOGGER.log(Level.WARNING, "Url {0} does not look like an update center:", siteUrl);
          }
          return updateSiteMetadataUrl;
      }
docs: URL which exposes the metadata location in a specific update site.
  @param downloadable, the downloadable id of a specific metatadata json (e.g. hudson.tasks.Maven.MavenInstaller.json)
  @return the location
  @since 2.20