fix(core/metrics): set target difficulty as single value (#4902)

Description
---
`base_node::blockchain::target_difficulty_sha` and `base_node::blockchain::target_difficulty_monero` are now single values at any point in time rather than qualified by block height. 


Motivation and Context
---
 I was not able to find a way (many months ago) to produce a performant non-timeseries graph that used height as the x-axis. The way I found tended to produce a lot of data on the endpoint side and crash grafana. We used a timeseries graph with an aggregation, producing an average difficulty over a time period. This PR allows for the precise difficulty w.r.t real time which I feel is better.

How Has This Been Tested?
---
Not tested, code compiles and this is how a lot of metric values work.

base_layer/core/src/base_node/comms_interface/inbound_handlers.rs

@@ -774,11 +774,11 @@ where B: BlockchainBackend + 'static
         fn update_target_difficulty(block: &ChainBlock) {
             match block.header().pow_algo() {
                 PowAlgorithm::Sha3 => {
-                    metrics::target_difficulty_sha(block.height())
+                    metrics::target_difficulty_sha()
                         .set(i64::try_from(block.accumulated_data().target_difficulty.as_u64()).unwrap_or(i64::MAX));
                 },
                 PowAlgorithm::Monero => {
-                    metrics::target_difficulty_monero(block.height())
+                    metrics::target_difficulty_monero()
                         .set(i64::try_from(block.accumulated_data().target_difficulty.as_u64()).unwrap_or(i64::MAX));
                 },
             }

base_layer/core/src/base_node/metrics.rs

@@ -25,38 +25,36 @@ use tari_common_types::types::FixedHash;
 use tari_metrics::{IntCounter, IntCounterVec, IntGauge, IntGaugeVec};
 use tari_utilities::hex::Hex;
 
-pub fn tip_height() -> IntGauge {
+pub fn tip_height() -> &'static IntGauge {
     static METER: Lazy<IntGauge> = Lazy::new(|| {
         tari_metrics::register_int_gauge("base_node::blockchain::tip_height", "The current tip height").unwrap()
     });
 
-    METER.clone()
+    &METER
 }
 
-pub fn target_difficulty_sha(height: u64) -> IntGauge {
-    static METER: Lazy<IntGaugeVec> = Lazy::new(|| {
-        tari_metrics::register_int_gauge_vec(
+pub fn target_difficulty_sha() -> &'static IntGauge {
+    static METER: Lazy<IntGauge> = Lazy::new(|| {
+        tari_metrics::register_int_gauge(
             "base_node::blockchain::target_difficulty_sha",
             "The current miner target difficulty for the sha3 PoW algo",
-            &["height"],
         )
         .unwrap()
     });
 
-    METER.with_label_values(&[&height.to_string()])
+    &METER
 }
 
-pub fn target_difficulty_monero(height: u64) -> IntGauge {
-    static METER: Lazy<IntGaugeVec> = Lazy::new(|| {
-        tari_metrics::register_int_gauge_vec(
+pub fn target_difficulty_monero() -> &'static IntGauge {
+    static METER: Lazy<IntGauge> = Lazy::new(|| {
+        tari_metrics::register_int_gauge(
             "base_node::blockchain::target_difficulty_monero",
             "The current miner target difficulty for the monero PoW algo",
-            &["height"],
         )
         .unwrap()
     });
 
-    METER.with_label_values(&[&height.to_string()])
+    &METER
 }
 
 pub fn reorg(fork_height: u64, num_added: usize, num_removed: usize) -> IntGauge {
@@ -153,7 +151,7 @@ pub fn rejected_local_blocks(height: u64, hash: &FixedHash) -> IntCounter {
     METER.with_label_values(&[&height.to_string(), &hash.to_hex()])
 }
 
-pub fn active_sync_peers() -> IntGauge {
+pub fn active_sync_peers() -> &'static IntGauge {
     static METER: Lazy<IntGauge> = Lazy::new(|| {
         tari_metrics::register_int_gauge(
             "base_node::sync::active_peers",
@@ -162,10 +160,10 @@ pub fn active_sync_peers() -> IntGauge {
         .unwrap()
     });
 
-    METER.clone()
+    &METER
 }
 
-pub fn utxo_set_size() -> IntGauge {
+pub fn utxo_set_size() -> &'static IntGauge {
     static METER: Lazy<IntGauge> = Lazy::new(|| {
         tari_metrics::register_int_gauge(
             "base_node::blockchain::utxo_set_size",
@@ -174,5 +172,5 @@ pub fn utxo_set_size() -> IntGauge {
         .unwrap()
     });
 
-    METER.clone()
+    &METER
 }