Metricbeat does not respect on output.file.permissions

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For confirmed bugs, please report:
- Version: 7.5.2
- Operating System: Centos 7.6
- Discuss Forum URL: https://discuss.elastic.co/t/metricbeat-does-not-respect-on-output-file-permissions/227325
- Steps to Reproduce:

My metricbeat config for file output is this:
```
output.file:
# Boolean flag to enable or disable the output module.
enabled: True

# Configure JSON encoding
#codec.json:
# Pretty-print JSON event
#pretty: false

# Configure escaping HTML symbols in strings.
#escape_html: false

# Path to the directory where to save the generated files. The option is
# mandatory.
path: "/var/log/metricbeat/probes"

# Name of the generated files. The default is `metricbeat` and it generates
# files: `metricbeat`, `metricbeat.1`, `metricbeat.2`, etc.
filename: metricbeat_probes.log

# Maximum size in kilobytes of each file. When this size is reached, and on
# every Metricbeat restart, the files are rotated. The default value is 10240
# kB.
rotate_every_kb: 10240

# Maximum number of files under path. When this number of files is reached,
# the oldest file is deleted and the rest are shifted from last to first. The
# default is 7 files.
number_of_files: 3

# Permissions to use for file creation. The default is 0600.
permissions: 0644
```

But the created files have 0640 permissions:
```
[root@elastic01 probes]# pwd
/var/log/metricbeat/probes
[root@elastic01 probes]# ls -l
total 68
-rw-r-----. 1 metricbeat metricbeat 68454 Apr 9 15:15 metricbeat_probes.log
```

Elastic already reproduced : Logfile:
```
2020-04-10T13:59:56.887-0600 INFO [file] fileout/file.go:100 Initialized file output. path=/Users/kaiyansheng/metricbeat-logs/metricbeat_probes.log max_size_bytes=10485760 max_backups=3 permissions=-rw-r--r--
```
Filesystem:
```
-rw-r----- 1 kaiyansheng staff 17K Apr 10 13:56 metricbeat_probes.log
```

Start in fileout/file.go around the initialization logged at line 100, using the provided output.file configuration and filesystem listing as the reproduction. Verify how permissions are applied when metricbeat_probes.log is created; done means the generated file uses the configured 0644 mode rather than 0640.

Written by the indexing model from the issue text.