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Loader for metrics implementation and a no-op implementation

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/*
 * Copyright (c) 2021, 2023 Oracle and/or its affiliates.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package io.helidon.metrics.api;

import java.lang.System.Logger.Level;
import java.util.List;
import java.util.ServiceLoader;
import java.util.StringJoiner;
import java.util.function.Predicate;
import java.util.function.Supplier;

import io.helidon.common.HelidonServiceLoader;
import io.helidon.metrics.spi.ExemplarService;

/**
 * Loads the {@link io.helidon.metrics.spi.ExemplarService} instance (if any) with the most urgent priority.
 */
class ExemplarServiceManager {

    static final String INACTIVE_LABEL = "";
    private static final System.Logger LOGGER = System.getLogger(ExemplarServiceManager.class.getName());
    private static final List EXEMPLAR_SERVICES = collectExemplarServices();
    private static final boolean IS_ACTIVE = !EXEMPLAR_SERVICES.isEmpty();
    private static final Supplier EXEMPLAR_SUPPLIER = () -> EXEMPLAR_SERVICES.stream()
            .map(ExemplarService::label)
            .filter(Predicate.not(String::isBlank))
            .collect(ExemplarServiceManager::labelsStringJoiner, StringJoiner::add, StringJoiner::merge)
            .toString();

    private ExemplarServiceManager() {
    }

    /**
     * Returns the current exemplar label (e.g., trace ID).
     *
     * @return exemplar string provided by the highest-priority service instance
     */
    static String exemplarLabel() {
        return IS_ACTIVE ? EXEMPLAR_SUPPLIER.get() : INACTIVE_LABEL;
    }

    /**
     * @return whether exemplar handling is active or not
     */
    static boolean isActive() {
        return IS_ACTIVE;
    }

    private static StringJoiner labelsStringJoiner() {
        // A StringJoiner that suppresses the prefix and suffix if no strings were added
        return new StringJoiner(",", "{", "}").setEmptyValue("");
    }

    private static List collectExemplarServices() {
        List exemplarServices =
                HelidonServiceLoader.create(ServiceLoader.load(ExemplarService.class)).asList();
        if (!exemplarServices.isEmpty()) {
            LOGGER.log(Level.INFO, "Using metrics ExemplarServices " + exemplarServices.toString());
        }

        return exemplarServices;
    }
}