There is an issue in the Grok Processor, where trace_match: true does not inject the _ingest._grok_match_index into the ingest-document when there is just one pattern provided. This is due to an optimization in the regex construction. This commit adds a check for when this is the case, and injects a static index value of "0", since there is only one pattern matched (at the first index into the patterns).
To make this clearer, more documentation was added to the grok-processor docs.
Fixes#21371.
04b712bdc5