@FunctionalInterface
public interface WebFluxTagsProvider {

  /**
   * Provides tags to be associated with metrics for the given {@code exchange}.
   * @param exchange the exchange
   * @param ex the current exception (may be {@code null})
   * @return tags to associate with metrics for the request and response exchange
   */
  Iterable<Tag> httpRequestTags(ServerWebExchange exchange, Throwable ex);

}

public class DefaultWebFluxTagsProvider implements WebFluxTagsProvider {

  @Override
  public Iterable<Tag> httpRequestTags(ServerWebExchange exchange,
      Throwable exception) {
    return Arrays.asList(WebFluxTags.method(exchange), WebFluxTags.uri(exchange),
        WebFluxTags.exception(exception), WebFluxTags.status(exchange),
        WebFluxTags.outcome(exchange));
  }

}

public final class WebFluxTags {

  private static final Tag URI_NOT_FOUND = Tag.of("uri", "NOT_FOUND");

  private static final Tag URI_REDIRECTION = Tag.of("uri", "REDIRECTION");

  private static final Tag URI_ROOT = Tag.of("uri", "root");

  private static final Tag URI_UNKNOWN = Tag.of("uri", "UNKNOWN");

  private static final Tag EXCEPTION_NONE = Tag.of("exception", "None");

  private static final Tag OUTCOME_UNKNOWN = Tag.of("outcome", "UNKNOWN");

  private static final Tag OUTCOME_INFORMATIONAL = Tag.of("outcome", "INFORMATIONAL");

  private static final Tag OUTCOME_SUCCESS = Tag.of("outcome", "SUCCESS");

  private static final Tag OUTCOME_REDIRECTION = Tag.of("outcome", "REDIRECTION");

  private static final Tag OUTCOME_CLIENT_ERROR = Tag.of("outcome", "CLIENT_ERROR");

  private static final Tag OUTCOME_SERVER_ERROR = Tag.of("outcome", "SERVER_ERROR");

  private WebFluxTags() {
  }

  public static Tag method(ServerWebExchange exchange) {
    return Tag.of("method", exchange.getRequest().getMethodValue());
  }

  public static Tag status(ServerWebExchange exchange) {
    HttpStatus status = exchange.getResponse().getStatusCode();
    if (status == null) {
      status = HttpStatus.OK;
    }
    return Tag.of("status", String.valueOf(status.value()));
  }

  public static Tag uri(ServerWebExchange exchange) {
    PathPattern pathPattern = exchange
        .getAttribute(HandlerMapping.BEST_MATCHING_PATTERN_ATTRIBUTE);
    if (pathPattern != null) {
      return Tag.of("uri", pathPattern.getPatternString());
    }
    HttpStatus status = exchange.getResponse().getStatusCode();
    if (status != null) {
      if (status.is3xxRedirection()) {
        return URI_REDIRECTION;
      }
      if (status == HttpStatus.NOT_FOUND) {
        return URI_NOT_FOUND;
      }
    }
    String path = getPathInfo(exchange);
    if (path.isEmpty()) {
      return URI_ROOT;
    }
    return URI_UNKNOWN;
  }

  private static String getPathInfo(ServerWebExchange exchange) {
    String path = exchange.getRequest().getPath().value();
    String uri = StringUtils.hasText(path) ? path : "/";
    return uri.replaceAll("//+", "/").replaceAll("/$", "");
  }

  public static Tag exception(Throwable exception) {
    if (exception != null) {
      String simpleName = exception.getClass().getSimpleName();
      return Tag.of("exception", StringUtils.hasText(simpleName) ? simpleName
          : exception.getClass().getName());
    }
    return EXCEPTION_NONE;
  }

  public static Tag outcome(ServerWebExchange exchange) {
    HttpStatus status = exchange.getResponse().getStatusCode();
    if (status != null) {
      if (status.is1xxInformational()) {
        return OUTCOME_INFORMATIONAL;
      }
      if (status.is2xxSuccessful()) {
        return OUTCOME_SUCCESS;
      }
      if (status.is3xxRedirection()) {
        return OUTCOME_REDIRECTION;
      }
      if (status.is4xxClientError()) {
        return OUTCOME_CLIENT_ERROR;
      }
      return OUTCOME_SERVER_ERROR;
    }
    return OUTCOME_UNKNOWN;
  }

}