Fix pipeline when user.name and postgresql.log.database with brackets

[kaiyan-sheng]

What does this PR do?

This PR is to add support for brackets around user.name and postgresql.log.database in ingest pipeline for parsing AWS RDS Postgresql logs.

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.

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• Start Time: 2022-07-22T18:56:50.199+0000

• Duration: 17 min 22 sec

Test stats 🧪

Test	Results
Failed	0
Passed	59
Skipped	0
Total	59

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (1/1)	💚
Files	100.0% (4/4)	💚 2.941
Classes	100.0% (4/4)	💚 2.941
Methods	93.333% (28/30)	👍 3.602
Lines	97.041% (164/169)	👍 6.459
Conditionals	100.0% (0/0)	💚