Downsampling is broken. Not sure why yet (likely caused by #97557). Invoking the downsample api results into:

```
org.elasticsearch.ElasticsearchSecurityException: no indices permissions found
        at org.elasticsearch.xpack.core.security.support.Exceptions.authorizationError(Exceptions.java:32) ~[?:?]
        at org.elasticsearch.xpack.core.security.authz.accesscontrol.SecurityIndexReaderWrapper.getIndicesAccessControl(SecurityIn
dexReaderWrapper.java:110) ~[?:?]
        at org.elasticsearch.xpack.core.security.authz.accesscontrol.SecurityIndexReaderWrapper.apply(SecurityIndexReaderWrapper.j
ava:76) ~[?:?]
        at org.elasticsearch.xpack.core.security.authz.accesscontrol.SecurityIndexReaderWrapper.apply(SecurityIndexReaderWrapper.j
ava:46) ~[?:?]
        at org.elasticsearch.index.shard.IndexShard.wrapSearcher(IndexShard.java:1590) ~[elasticsearch-8.11.0-SNAPSHOT.jar:?]
        at org.elasticsearch.index.shard.IndexShard.wrapSearcher(IndexShard.java:1561) ~[elasticsearch-8.11.0-SNAPSHOT.jar:?]
        at org.elasticsearch.index.engine.Engine$SearcherSupplier.acquireSearcher(Engine.java:1321) ~[elasticsearch-8.11.0-SNAPSHO
T.jar:?]
        at org.elasticsearch.index.engine.Engine.acquireSearcher(Engine.java:774) ~[elasticsearch-8.11.0-SNAPSHOT.jar:?]
        at org.elasticsearch.index.shard.IndexShard.acquireSearcher(IndexShard.java:1549) ~[elasticsearch-8.11.0-SNAPSHOT.jar:?]
        at org.elasticsearch.xpack.downsample.DownsampleShardIndexer.<init>(DownsampleShardIndexer.java:115) ~[?:?]
        at org.elasticsearch.xpack.downsample.DownsampleShardPersistentTaskExecutor.startDownsampleShardIndexer(DownsampleShardPersistentTaskExecutor.java:111) ~[?:?]
        at org.elasticsearch.xpack.downsample.DownsampleShardPersistentTaskExecutor$1.doRun(DownsampleShardPersistentTaskExecutor.java:87) ~[?:?]
        at org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingAbstractRunnable.doRun(ThreadContext.java:983) ~[elasticsearch-8.11.0-SNAPSHOT.jar:?]
        at org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:26) ~[elasticsearch-8.11.0-SNAPSHOT.jar:?]
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144) ~[?:?]
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642) ~[?:?]
        at java.lang.Thread.run(Thread.java:1623) ~[?:?]
```
x-pack/plugin/downsample/src/main/java/org/elasticsearch/xpack/downsample/DownsampleShardIndexer.java
final String[] metrics,
        final String[] labels,
        final DownsampleShardPersistentTaskState state
    ) {
        this.task = task;
        this.client = client;
        this.indexShard = indexService.getShard(shardId.id());
        this.downsampleIndex = downsampleIndex;
        this.searcher = indexShard.acquireSearcher("downsampling");
        this.state = state;
        Closeable toClose = searcher;
        try {
            this.searchExecutionContext = indexService.newSearchExecutionContext(
                indexShard.shardId().id(),
                0,
                searcher,
                () -> 0L,
                null,
                Collections.emptyMap()
            );
            this.timestampField = (DateFieldMapper.DateFieldType) searchExecutionContext.getFieldType(config.getTimestampField());
            this.timestampFormat = timestampField.docValueFormat(null, null);
            this.rounding = config.createRounding();
            List<FieldValueFetcher> fetchers = new ArrayList<>(metrics.length + labels.length);
            fetchers.addAll(FieldValueFetcher.create(searchExecutionContext, metrics));
            fetchers.addAll(FieldValueFetcher.create(searchExecutionContext, labels));
            this.fieldValueFetchers = Collections.unmodifiableList(fetchers);
            toClose = null;
        } finally {
            IOUtils.closeWhileHandlingException(toClose);