ElasticsearchIO performs 0 division on DirectRunner

- Environment configuration

In my company we use [Elasticsearch cross cluster](https://www.elastic.co/guide/en/elasticsearch/reference/current/modules-cross-cluster-search.html#ccs-supported-apis) setup for search. Cluster version is 7.9.1.

I intended to use ElasticsearchIO for reading application logs and subsequently producing some aggregated data.
- Problem description
- In cross cluster ES setup, there is no `//_stats` API available, so it is not possible to compute [ElasticsearchIO#getEstimatedSizeBytes](https://github.com/apache/beam/blob/master/sdks/java/io/elasticsearch/src/main/java/org/apache/beam/sdk/io/elasticsearch/ElasticsearchIO.java#L692) properly.
- `statsJson` returned by the cluster looks like this:
> Unknown macro: \{ "_shards" }
>
> _all" :
> nknown macro: \{ "primaries" }
>
> total" : \{ }
> ,
> indices" : \{ }
>
> - That means that `totalCount` value cannot be parsed from the json and is thus set to `0`.
- Which means that `estimatedByteSize` value [is set to 1](https://github.com/apache/beam/blob/master/sdks/java/io/elasticsearch/src/main/java/org/apache/beam/sdk/io/elasticsearch/ElasticsearchIO.java#L707) (Which itself is a workaround for similar issue.)
- `ElasticsearchIO#getEstimatedSizeBytes` is used in [BoundedReadEvaluatorFactory#getInitialInputs](https://github.com/apache/beam/blob/master/runners/direct-java/src/main/java/org/apache/beam/runners/direct/BoundedReadEvaluatorFactory.java#L212) which does not check the value and performs division of two `long` values, which of course results in `0` for any `targetParallelism > 1`.
- Then [ElasticsearchIO#split](https://github.com/apache/beam/blob/master/sdks/java/io/elasticsearch/src/main/java/org/apache/beam/sdk/io/elasticsearch/ElasticsearchIO.java#L665) is called with `indexSize = 1` and `desiredBundleSizeBytes = 1`. Which sets `nbBundlesFloat` value to infinity.
- Even though the number of bundles is ceiled at `1024`, reading from 1024 BoundedElasticsearchSources concurrently makes the ElasticsearchIO virtually impossible to use on direct runner.

- Resolution suggestion

I still haven't tested reading from ElasticsearchIO on proper runner (we use flink 1.10.2), so I cannot either confirm or deny its functionality on our elastic setup. At the moment I'm just suggesting few checks of input values so the zero division and unnecessary parallelism problems are eliminated on direct runner.

Imported from Jira [BEAM-10945](https://issues.apache.org/jira/browse/BEAM-10945). Original Jira may contain additional context.
Reported by: DraCzech.