Let the JSON formatter include a metric's data_names. For that purpose, let the metric store provide a reference to the timeseries information object (to be populated on read). The JSON formatter compiles a list of unique data_names and outputs that alongside the other metric metadata.
Handle last_update/interval in the core rather than in memstore. For this purpose, let the sdb_plugin_store_* functions fetch the latest revision of the object to determine the last update interval and last recorded interval. The memstore now expects both values to be passed in whenever storing/updating an object.