Ignore docker kill events in add_docker_metadata

[exekias]

Docker kill events are caused by any signal sent to the container, not only a SIGKILL, we can safely ignore all of them, as die will always happen for the case we are really interested in.

Current code is removing forgetting metadata for containers that get a kill event, this PR fixes that.

6.1 and master are not affected, as kill was ignored already: https://github.com/elastic/beats/blob/6.1/libbeat/common/docker/watcher.go#L237

Context on the issue: https://discuss.elastic.co/t/filebeats-docker-metadata-watcher-losing-container-from-its-list-somehow/109897

[codecov]

Codecov Report

Merging #5788 into 6.0 will decrease coverage by 0.9%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##              6.0    #5788      +/-   ##
==========================================
- Coverage   63.36%   62.45%   -0.91%     
==========================================
  Files         610      614       +4     
  Lines       39337    41120    +1783     
==========================================
+ Hits        24924    25680     +756     
- Misses      11953    12956    +1003     
- Partials     2460     2484      +24

Impacted Files	Coverage Δ
libbeat/processors/add_docker_metadata/watcher.go	80.8% <100%> (ø)	⬆️
metricbeat/module/nginx/stubstatus/data.go	58.57% <0%> (-28.67%)	⬇️
metricbeat/module/redis/info/data.go	75% <0%> (-25%)	⬇️
metricbeat/module/haproxy/stat/data.go	75% <0%> (-25%)	⬇️
metricbeat/module/redis/keyspace/data.go	75.67% <0%> (-24.33%)	⬇️
metricbeat/module/jolokia/jmx/config.go	76.47% <0%> (-23.53%)	⬇️
metricbeat/module/docker/helper.go	61.76% <0%> (-22.24%)	⬇️
metricbeat/module/prometheus/stats/stats.go	61.29% <0%> (-21.32%)	⬇️
metricbeat/module/haproxy/info/data.go	44.82% <0%> (-20.18%)	⬇️
metricbeat/module/docker/container/data.go	80.76% <0%> (-19.24%)	⬇️
... and 182 more

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