Go: document that users should check Enabled when making sychronous metric API calls

[dashpole]

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Part of open-telemetry/opentelemetry-go#7800

The Performance Optimization for Synchronous Instruments section will be expanded later to include pooling, and precomputing attribute sets.

cc @open-telemetry/go-approvers

[tiffany76]

/fix:format

[otelbot-docs]

✅ fix:format applied successfully in run 24370065504.