### Description

Before ES 7.x it was possible to provide a list of fields by URL or BODY request for field_caps API.
https://www.elastic.co/guide/en/elasticsearch/reference/6.8/search-field-caps.html
`POST _field_caps
{
   "fields" : ["rating"]
}`

Starting from 7.x this feature was removed, and only one possible way it uses URI but in this case, it is possible to have an error 414 (Request-URI Too Long).

I would like to return this feature back.
server/src/test/java/org/elasticsearch/action/fieldcaps/FieldCapabilitiesRequestTests.java
return FieldCapabilitiesRequest::new;
    }
    @Override
    protected FieldCapabilitiesRequest mutateInstance(FieldCapabilitiesRequest instance) throws IOException {
        List<Consumer<FieldCapabilitiesRequest>> mutators = new ArrayList<>();
        mutators.add(request -> {
            String[] fields = ArrayUtils.concat(request.fields(), new String[] { randomAlphaOfLength(10) });
            request.fields(fields);
        });
        mutators.add(request -> {
            String[] indices = ArrayUtils.concat(instance.indices(), generateRandomStringArray(5, 10, false, false));
            request.indices(indices);
        });
        mutators.add(request -> {
            IndicesOptions indicesOptions = randomValueOtherThan(
                request.indicesOptions(),
                () -> IndicesOptions.fromOptions(randomBoolean(), randomBoolean(), randomBoolean(), randomBoolean())
            );
            request.indicesOptions(indicesOptions);
        });
        mutators.add(request -> request.setMergeResults(request.isMergeResults() == false));
        mutators.add(request -> request.includeUnmapped(request.includeUnmapped() == false));
        mutators.add(request -> request.nowInMillis(request.nowInMillis() != null ? request.nowInMillis() + 1 : 1L));
        mutators.add(
            request -> request.indexFilter(request.indexFilter() != null ? request.indexFilter().boost(2) : QueryBuilders.matchAllQuery())
        );
        mutators.add(request -> request.runtimeFields(Collections.singletonMap("other_key", "other_value")));
        mutators.add(request -> request.filters(request.filters().length == 0 ? new String[] { "-metadata" } : Strings.EMPTY_ARRAY));
        mutators.add(request -> request.types(request.types().length == 0 ? new String[] { "keyword" } : Strings.EMPTY_ARRAY));
        FieldCapabilitiesRequest mutatedInstance = copyInstance(instance);