### Description

I was just looking at some Elasticsearch that had lots of "Improve "reloading search analyzers" logs coming from `MapperService` at the INFO level. In my particular case, it was being emitted because analyzers are reloaded as part of doing a recovery. This log is not really helpful as-is, I think we need to do the following:
 - Switch the level to DEBUG, it's not important enough to warrant an INFO log.
 - Add which index reloaded its search analyzers to the log.

What do you think?
server/src/main/java/org/elasticsearch/index/mapper/MapperService.java
/**
     * The reason why a mapping is being merged.
     */
    public enum MergeReason {
        /**
         * Pre-flight check before sending a mapping update to the master
         */
        MAPPING_UPDATE_PREFLIGHT,
        /**
         * Create or update a mapping.
         */
        MAPPING_UPDATE,
        /**
         * Merge mappings from a composable index template.
         */
        INDEX_TEMPLATE,
        /**
         * Recovery of an existing mapping, for instance because of a restart,
         * if a shard was moved to a different node or for administrative
         * purposes.
         */
        MAPPING_RECOVERY
    }
    public static final String SINGLE_MAPPING_NAME = "_doc";