Update Metrics section in documentation

[barchetta]

Environment Details

• Helidon Version: 0.10.0
• Helidon SE

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Problem Description

The Metrics and Health Support in our documentation needs to be updated. It just mentions PrometheusSupport and not the new metrics implementation. The new implementation is based on Helidon's support for MicroProfile Metrics and can be used in a Helidon SE application. See the ToDos example. Specifically the “demo-frontend”. It performs registration in “Main” class, line 150 and usage in “TodosHandler”