chore: Observability improvements to node logs and metrics | NPG-7984 (#542)

# Description

Update a couple of log messages to clarify fragments rejected from the
mempool.
Convert `lastBlockTx` prometheus metric from being a gauge into counter.
Add docstrings to clarify the meaning of prometheus metrics related to
transactions.

## Type of change

Please delete options that are not relevant.

- [x] New feature (non-breaking change which adds functionality)

## Checklist

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [x] I have commented my code, particularly in hard-to-understand areas
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

Author: saibatizoku

src/jormungandr/jormungandr/src/fragment/pool.rs

@@ -96,12 +96,12 @@ impl Pool {
         id: FragmentId,
     ) -> Result<(), FragmentRejectionReason> {
         if self.logs.exists(id) {
-            tracing::debug!("fragment is already logged");
+            tracing::debug!("fragment is already logged, excluding from the pool");
             return Err(FragmentRejectionReason::FragmentAlreadyInLog);
         }
 
         if !is_fragment_valid(fragment) {
-            tracing::debug!("fragment is invalid, not including to the pool");
+            tracing::debug!("fragment is invalid, excluding from the pool");
             return Err(FragmentRejectionReason::FragmentInvalid);
         }
 
@@ -122,7 +122,7 @@ impl Pool {
             }
         }
 
-        tracing::debug!("including fragment to the pool");
+        tracing::debug!("fragment is valid for pool entry");
         Ok(())
     }
 

src/jormungandr/jormungandr/src/metrics/backends/prometheus_exporter.rs

@@ -17,9 +17,13 @@ type UIntGauge = GenericGauge<AtomicU64>;
 pub struct Prometheus {
     registry: Registry,
 
+    // Total number of tx accepted for a given block
     tx_recv_cnt: IntCounter,
+    // Total number of tx rejected for a given block
     tx_rejected_cnt: IntCounter,
+    // Mempool usage ratio for a given block
     mempool_usage_ratio: Gauge,
+    // Total number of tx in the mempool for a given block
     mempool_tx_count: UIntGauge,
     votes_casted_cnt: IntCounter,
     block_recv_cnt: IntCounter,
@@ -28,7 +32,8 @@ pub struct Prometheus {
     peer_available_cnt: UIntGauge,
     peer_total_cnt: UIntGauge,
     slot_start_time: UIntGauge,
-    block_tx_count: UIntGauge,
+    // Total number of tx for a given block
+    block_tx_count: IntCounter,
     block_input_sum: UIntGauge,
     block_fee_sum: UIntGauge,
     block_content_size: UIntGauge,
@@ -116,7 +121,7 @@ impl Default for Prometheus {
         registry
             .register(Box::new(slot_start_time.clone()))
             .unwrap();
-        let block_tx_count = UIntGauge::new("lastBlockTx", "lastBlockTx").unwrap();
+        let block_tx_count = IntCounter::new("lastBlockTx", "lastBlockTx").unwrap();
         registry.register(Box::new(block_tx_count.clone())).unwrap();
         let block_input_sum = UIntGauge::new("lastBlockInputTime", "lastBlockInputTime").unwrap();
         registry
@@ -280,7 +285,7 @@ impl MetricsBackend for Prometheus {
             .expect("should be good");
 
         self.votes_casted_cnt.inc_by(votes_casted);
-        self.block_tx_count.set(block_tx_count);
+        self.block_tx_count.inc_by(block_tx_count);
         self.block_input_sum.set(block_input_sum.0);
         self.block_fee_sum.set(block_fee_sum.0);
         self.block_content_size