[cisco_ios] Improve hostname parsing

[mjwolf]

Proposed commit message

Improves parsing of hostnames, to allow hostnames that begin with digits. This is not strictly allowed by the Cisco IOS specification, but it is commonly allowed on systems. So support for it should be added.

Due to the limitations of the grok parser's regex support, and the many possible formats of Cisco IOS logs, this required using a script to do some additional processing. With leading digits allowed in hostnames, they can appear to be sequence numbers, which can appear in the exact same location in the logs as hostname, and are also all digits. Forward look-ahead to check that there's any letter in the hostname pattern doesn't work to solve this, since the look-ahead cannot be contained to groups. Instead the script used to find this case, after grok.

Additionally there is another log format, when using year-first date stamps, the year can be mistaken sequence number. This also removes sequence number, if it appears that it was mistakenly taken from a timestamp.

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.
• I have verified that any added dashboard complies with Kibana's Dashboard good practices

[elasticmachine]

💚 Build Succeeded

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• Commit: cde4c94

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Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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[elasticmachine]

Pinging @elastic/sec-deployment-and-devices (Team:Security-Deployment and Devices)