Add instrumentation for value-based entities (Entity)

[Guy Youansi]

Add some metrics for value-based entities using the already available prometheus client.
Metrics should be tagged by the user function entity name (such as "shopping-cart") so they're aggregated metrics for an entity type on a node.

Count metrics include:

• gauge: entity instances currently active
• count: entity instances that have activated
• count: entity instances that have passivated
• count: commands received for an entity
• count: commands stashed for an entity
• count: commands processed for an entity
• count: load operations for an entity
• count: state sizes (bytes) loaded for an entity
• count: persist operations for an entity
• count: state sizes (bytes) persisted for an entity
• count: delete operations for an entity

Timing metrics include:

• histogram: load operations duration for an entity
• histogram: persist operations duration for an entity
• histogram: delete operations duration for an entity

Please see #220