Small inconsistency in stats aggregation rest output vs. java api

[cbuescher]

While working on adding parsing for the stats aggregation I noticed a small difference between the output of the formatted
count value in the Java API vs. the rest output. The xContent output of InternalStats writes formatted "as_string"-fields for
all calculated statistics (min/max/avg/sum) except for the count. Using the Java API and getCountAsString() (which is also
part of the Stats interface) however, it is possible to retrieve a formatted value that uses the configured formatter
(e.g. a decimal or date formatter).

It would be good to either make this consitent by either adding the count_as_string output to rest or change getCountAsString() so it
doesn't use the formatter and instead only returns the long as a string. Another option, at least for the parsing in #24239, would be to
remove getCountAsString() from the Stats interface.

Relates to #24239

[cbuescher]

@colings86 do you think we can add this field in v6.0.0 or do you think this needs fixing at all? We can also ignore it as an edge case but if possible I'd like to fix it.

[colings86]

I don't think we should add a count_as_string field here as I don't think it would make sense since trying to format a count of e.g. 3 as a date etc. does not produce a meaningful result.

IMO we should remove the getCountAsString() field form the java class in 6.0.0 and deprecate it in favour of String.valueOf(getCount) in 5.x