BloombergLP::balm::MetricsManager::registerCollectionCallback

Register the specified callback to collect records for the category having the specified categoryName, 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 the identified category each time the category is published or collected, even if the category is disabled. If the publish or collectSample method is called on the identified category, and the 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 the one identified by categoryName, or if categoryName is not null-terminated. 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.