Metricbeat: reduce disk usage in default configuration

[tsg]

Meta ticket for several improvements we plan for Metricbeat in order to reduce the storage space requirements in its default configuration.

• Metricbeat: store only top N processes by CPU/memory #4127 Add a feature to allow collecting only the first top N processes by CPU and/or memory, instead of all of them.
• Add process summary metrics #4231 Add new metrics for the total process counts and use those in dashboards
• Template setting in the configuration file #4284 Make it possible to set "shards" and "replicas" from the metricbeat YAML file.
• Change Metricbeat default config to reduce disk space #4329 Set the default number of shards to 1.
• Add option to adjust _source options + improvements #4317 Make it easily possible to disable _source, but leave it on by default.
• Change Metricbeat default config to reduce disk space #4329 Enable best compression by default.
• Change Metricbeat default config to reduce disk space #4329 Reduce the collection interval for filesystem statistics and disable fsstat by default. Set it to 1m.
• Change Metricbeat default config to reduce disk space #4329 Filter out several filesystem types by mount point (e.g. proc, tmpfs, dev)
• Filter out logical devices from the diskio stats

We target these changes for the 6.0 release.