cmd/apmtool: add wait flags for `generate-trace`

We are missing the final doc indexing assertion step in the `generate-trace` command. Following is the quote from the [original issue](https://github.com/elastic/apm-tools/issues/1):

> flags for waiting for events to be indexed, using the same functionality underlying https://github.com/elastic/apm-tools/issues/2
> The search performed in the final step of waiting for events to be indexed should include two [runtime fields](https://www.elastic.co/guide/en/elasticsearch/reference/current/search-your-data.html#run-search-runtime-fields) (names TBD):
>
> event.ingested - timestamp
> event.ingested - event.received