Implement better cogs accounting for worker time usage

[markstory]

To give better visibility into the costs of taskbroker/workers we should record usage in finer detail.

Workers

Workers should already be measuring task execution time. We should use sentry.usageaccountant to record time spent on tasks per app_feature. We'll need a way to map between namespaces and app_features. Perhaps that could be metadata that is included in the TaskNamespaces.

Brokers

Brokers could periodically emit task counts for each namespace they are handling. Again we'd need a map between namespaces and app_features. The https://github.com/getsentry/rust-usage-accountant library can be used to record and publish usage results.

[markstory]

Recommended labeling for brokers & workers:

• app_feature: the same as the corresponding celery task
• system: task_broker
• subsystem: broker / worker.

As brokers ingest messages from Kafka we could generate counts of activations by app_feature and emit those as kafka messages using usage-accountant. This implies that there is a configuration map of namespace : app_feature within broker configuration.