The char filter replaces the previous default of `first_non_blank_line`.
`first_non_blank_line` worked well to figure out what line had characters at all, but log lines
like the following were handled poorly:
```
--------------------------------------------------------------------------------
Alias 'foo' already exists and this prevents setting up ILM for logs
--------------------------------------------------------------------------------
```
When combined with the `ml_standard` tokenizer, the first line was used:
```
--------------------------------------------------------------------------------
```
This has no valid tokens for our standard tokenizer. Consequently, no tokens were found by `ml_standard` tokenizer.
The new filter, `first_line_with_letters`, returns the first line with any letter character (e.g. `Character#isLetter` returns true).
Given the previously poorly handled log, when combining with our `ml_standard` tokenizer, we get the following, more appropriate, tokens:
```
"tokens" : ["Alias", "foo", "already", "exists", "and", "this", "prevents", "setting", "up", "ILM", "for", "logs"]
```
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