[edge] Clarify the distinction between static and dynamic metrics

[amrc-benmorrow]

More and more I am finding we need to know which device metrics are static, and may not change except on rebirth, and which are dynamic and may be updated by DATA. We need to clarify this.

• We need a standard Property stating whether a metric is static or dynamic.
• The config file needs to be explicit about this.
• Some dynamic metrics have a meaningful 'resting' value, some do not. We need to support publishing nulls in BIRTHs for dynamic metrics we don't have a value for yet.