fix(serializers.splunkmetric): Fix TOML option name for multi-metric

[srebhan]

• Updated associated README.md.
• Wrote appropriate unit tests.
• Pull request title or commits are in conventional commit format

resolves #13510

In the course of migrating serializers to the new-style framework TOML annotations where created based in the Config structure in registry.go. However, there are subtle differences between those and the actual option parsing that was done in config.go. This is one instance of such a difference.

The present PR reverts the TOML option back to splunkmetric_multimetric as used before v1.27.0.

[telegraf-tiger]

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