Constant DYN_ENABLE_POLL_HISTOGRAM 

pub const DYN_ENABLE_POLL_HISTOGRAM: &str = "DYN_ENABLE_POLL_HISTOGRAM";

Enable Tokio task poll-time histogram (calls enable_metrics_poll_time_histogram on builder). Set to “1”, “true”, or “yes” to enable. Adds ~2× overhead of Instant::now() per task poll.