message: [txpool]: add namespace
diff: use once_cell::sync::Lazy;
  -use prometheus::{HistogramVec, IntCounterVec, IntGauge, IntGaugeVec};
  +use prometheus::{HistogramOpts, HistogramVec, IntCounterVec, IntGauge, IntGaugeVec, Opts};
  /// Counter of txn status in tx_pool
  pub static TXN_STATUS_COUNTERS: Lazy<IntCounterVec> = Lazy::new(|| {
  - register_int_counter_vec!(
  + let opts = Opts::new(
  "txpool_txn_stats",
  "Counters of how many txn's stats in tx pool",
  - &["status"]
  )
  - .unwrap()
  + .namespace("starcoin");
  + register_int_counter_vec!(opts, &["status"]).unwrap()
  });
  pub static TXPOOL_TXNS_GAUGE: Lazy<IntGauge> = Lazy::new(|| {
  - register_int_gauge!("txpool_txn_nums", "Counter of how many txns in txpool").unwrap()
  + let opts =
  + Opts::new("txpool_txn_nums", "Counter of how many txns in txpool").namespace("starcoin");
  + register_int_gauge!(opts).unwrap()
  });
  pub static TXPOOL_STATUS_GAUGE_VEC: Lazy<IntGaugeVec> = Lazy::new(|| {
  - register_int_gauge_vec!("txpool_status", "Gauge of pool status", &["name"]).unwrap()
  + let opts = Opts::new("txpool_status", "Gauge of pool status").namespace("starcoin");
  + register_int_gauge_vec!(opts, &["name"]).unwrap()
  });
  pub static TXPOOL_SERVICE_HISTOGRAM: Lazy<HistogramVec> = Lazy::new(|| {
  - register_histogram_vec!("txpool_service", "Histogram of txpool service", &["api"]).unwrap()
  + let opts =
  + HistogramOpts::new("txpool_service", "Histogram of txpool service").namespace("starcoin");
  + register_histogram_vec!(opts, &["api"]).unwrap()
  });
  