[hashicorp_vault] Clarify dynamic mapping for hashicorp_vault.metrics

[andrewkroh]

What does this PR do?

Remove named mappings and depend on the dynamic mappings. Dynamic mappings
were already present, but elastic-package doesn't honor those during field validation.
So this specifies a hashicorp_vault.metrics.*.* mapping for the express purpose of making
elastic-package field validation pass.

Fixes #2898

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.

Related issues

• Closes [hashicorp_vault] undefined metric fields detected by tests #2898

[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[elasticmachine]

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• Start Time: 2022-04-04T20:49:55.123+0000

• Duration: 17 min 38 sec

Test stats 🧪

Test	Results
Failed	0
Passed	18
Skipped	0
Total	18

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