Extend filebeat logstash patterns for the haproxy modul

[itsx]

Enhancement:
Enhance Logstash patterns in the haproxy module to match yet another specific kind of a logline (example of the logline is below).

Specific use case for the enhancement:
We are using ELK stack to collect logs. In our setup we don't use haproxy module, using our own filebeat and logstash configs.
But we have examined and used logstash grok patterns in a haproxy module to have rather complete cover of possible haproxy loglines.

After some time we spotted a couple of loglines, which were not matched and generated _grokparsefailure tag.
Apparently this logline correspond to a haproxy connection/availability problem and is quite uncommon.

In our setup we are using all grok patterns from the haproxy module with one added pattern to catch this logline (pattern below).

HAProxy version:
HA-Proxy version 1.8.8-1ubuntu0.10 2020/04/03
Unmatched logline:
Sep 21 14:54:37 api-lb1 haproxy[17648]: Server api-backend/api3 is DOWN, reason: Layer4 connection problem, info: "Connection refused", check duration: 0ms. 2 active and 0 backup servers left. 0 sessions active, 0 requeued, 0 remaining in queue.
Used grok patterns from a haproxy module:

beats/filebeat/module/haproxy/log/ingest/pipeline.yml

Line 10 in 829c3b7

- '%{HAPROXY_DATE:haproxy.request_date} %{IPORHOST:haproxy.source} %{PROG:process.name}(?:\[%{POSINT:process.pid:long}\])?:

Our added Logstash grok pattern:
%{SYSLOGTIMESTAMP:[@metadata][timestamp]} %{GREEDYDATA} %{NOTSPACE:process.name}\[%{NUMBER:process.pid}\]: Server %{NOTSPACE:haproxy.backend_name}/%{NOTSPACE:haproxy.server_name} %{GREEDYDATA}

[elasticmachine]

Pinging @elastic/integrations-services (Team:Services)

[botelastic]

Hi!
We just realized that we haven't looked into this issue in a while. We're sorry!

We're labeling this issue as Stale to make it hit our filters and make sure we get back to it as soon as possible. In the meantime, it'd be extremely helpful if you could take a look at it as well and confirm its relevance. A simple comment with a nice emoji will be enough :+1.
Thank you for your contribution!