The warning header used by Elasticsearch for delivering deprecation
warnings has a specific format (RFC 7234, section 5.5). The format
specifies that the warning header should be of the form
warn-code warn-agent warn-text [warn-date]
Here, the warn-code is a three-digit code which communicates various
meanings. The warn-agent is a string used to identify the source of the
warning (either a host:port combination, or some other identifier). The
warn-text is quoted string which conveys the semantic meaning of the
warning. The warn-date is an optional quoted date that can be in a few
different formats.
This commit corrects the warning header within Elasticsearch to follow
this specification. We use the warn-code 299 which means a
"miscellaneous persistent warning." For the warn-agent, we use the
version of Elasticsearch that produced the warning. The warn-text is
unchanged from what we deliver today, but is wrapped in quotes as
specified (this is important as a problem that exists today is that
multiple warnings can not be split by comma to obtain the individual
warnings as the warnings might themselves contain commas). For the
warn-date, we use the RFC 1123 format.
Relates #23275
577e6a5e14