### 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
throw new MapperParsingException("cannot merge mapping sources with different roots");
        }
        return (mergedRawMapping != null) ? doMerge(type, reason, mergedRawMapping) : null;
    }
    /**
     * A {@link org.elasticsearch.common.xcontent.XContentHelper.CustomMerge} for raw map merges that are suitable for index/field mappings.
     * The default raw map merge algorithm doesn't override values - if there are multiple values for a key, then:
     * <ul>
     *     <li> if the values are of map type, the old and new values will be merged recursively
     *     <li> otherwise, the original value will be maintained
     * </ul>
     * When merging field mappings, we want something else. Specifically:
     * <ul>
     *     <li> within field mappings node (which is nested within a {@code properties} node):
     *     <ul>
     *         <li> if both the base mapping and the mapping to merge into it are of mergeable types (e.g {@code object -> object},
     *         {@code object -> nested}), then we only want to merge specific mapping entries:
     *         <ul>
     *             <li> {@code properties} node - merging fields from both mappings
     *             <li> {@code subobjects} entry - since this setting affects an entire subtree, we need to keep it when merging
     *         </ul>
     *         <li> otherwise, for any couple of non-mergeable types ((e.g {@code object -> long}, {@code long -> long}) - we just want
     *         to replace the entire mappings subtree, let the last one win
     *     </ul>
     *     <li> any other map values that are not encountered within a {@code properties} node (e.g. "_doc", "_meta" or "properties"
     *     itself) - apply recursive merge as the default algorithm would apply
     *     <li> any non-map values - override the value of the base map with the value of the merged map
     * </ul>
     */
    private static class RawFieldMappingMerge implements XContentHelper.CustomMerge {