BloombergLP::balm::MetricsManager::registerCollectionCallback

Register the specified callback to collect records for the specified category, and return an opaque integer handle that can be used later to remove the callback. The supplied callback will be called to collect metrics from category each time category is published, even if category is disabled (i.e., category‐>enabled() is false). If the publish method is called on category and category is disabled, the callback will be invoked so clients may update any internal state, but the collected metrics will be ignored. Clients that wish to avoid overhead associated with collecting metrics for a disabled category must test (within their code) whether the category is disabled. The behavior is undefined if the callback returns metrics belonging to metric categories other than category. Note that the same callback can be registered for a category multiple times, and that registering the same callback multiple times will result in an equal number of invocations of the callback when publishing the category.