add comment explaining why the metric example exists

Author: spiral-ladder

examples/metrics_era.zig

@@ -5,6 +5,9 @@
 //! Run a Prometheus instance to see the data visually.
 //!
 //! To run this example, run `zig build run:metrics_era`.
+//!
+//! Note: this example is mainly meant to test that our metrics is working; realistically we do not need
+//! such an example. The bulk of this code should be moved to our beacon node implementation once it's ready.
 
 const download_era_options = @import("download_era_options");
 const era = @import("era");

src/state_transition/epoch/process_epoch.zig

@@ -2,6 +2,7 @@ const std = @import("std");
 const CachedBeaconState = @import("../cache/state_cache.zig").CachedBeaconState;
 const Timer = std.time.Timer;
 const metrics = @import("../metrics.zig");
+
 const ForkSeq = @import("config").ForkSeq;
 const EpochTransitionCache = @import("../cache/epoch_transition_cache.zig").EpochTransitionCache;
 const processJustificationAndFinalization = @import("./process_justification_and_finalization.zig").processJustificationAndFinalization;
@@ -90,7 +91,6 @@ pub fn processEpoch(allocator: std.mem.Allocator, cached_state: *CachedBeaconSta
 
     if (state.forkSeq().gte(.fulu)) {
         timer = try Timer.start();
-        timer = try metrics.state_transition.epoch_transition_step.time();
         try processProposerLookahead(allocator, cached_state, cache);
         try observeEpochTransitionStep(.{ .step = .process_proposer_lookahead }, timer.read());
     }

src/state_transition/metrics.zig

@@ -2,7 +2,7 @@ const std = @import("std");
 const Allocator = std.mem.Allocator;
 const m = @import("metrics");
 
-const CachedBeaconStateAllForks = @import("cache/state_cache.zig").CachedBeaconStateAllForks;
+const CachedBeaconState = @import("cache/state_cache.zig").CachedBeaconState;
 
 /// Defaults to noop metrics, making this safe to use whether or not `metrics.init` is called.
 pub var state_transition = m.initializeNoop(Metrics);
@@ -84,25 +84,29 @@ const Metrics = struct {
     const PreStateClonedCount = m.Histogram(u32, &.{ 1, 2, 5, 10, 50, 250 });
     const ProposerRewardsGauge = m.GaugeVec(u64, ProposerRewardLabel);
 
-    pub fn onStateClone(self: *Metrics, state: *CachedBeaconStateAllForks, source: StateCloneSource) !void {
-        try if (state.state.balances().items.len > 0)
+    pub fn onStateClone(self: *Metrics, state: *CachedBeaconState, source: StateCloneSource) !void {
+        var balances = try state.state.balances();
+        try if (try balances.length() > 0)
             self.pre_state_balances_nodes_populated_hit.incr(.{ .source = source })
         else
             self.pre_state_balances_nodes_populated_miss.incr(.{ .source = source });
 
-        try if (state.state.validators().items.len > 0)
+        var validators = try state.state.validators();
+        try if (try validators.length() > 0)
             self.pre_state_validators_nodes_populated_hit.incr(.{ .source = source })
         else
             self.pre_state_validators_nodes_populated_miss.incr(.{ .source = source });
     }
 
-    pub fn onPostState(self: *Metrics, state: *CachedBeaconStateAllForks) void {
-        if (state.state.balances().items.len > 0)
+    pub fn onPostState(self: *Metrics, state: *CachedBeaconState) !void {
+        var balances = try state.state.balances();
+        if (try balances.length() > 0)
             self.post_state_balances_nodes_populated_hit.incr()
         else
             self.post_state_balances_nodes_populated_miss.incr();
 
-        if (state.state.validators().items.len > 0)
+        var validators = try state.state.validators();
+        if (try validators.length() > 0)
             self.post_state_validators_nodes_populated_hit.incr()
         else
             self.post_state_validators_nodes_populated_miss.incr();

src/state_transition/state_transition.zig

@@ -172,13 +172,12 @@ pub fn stateTransition(
     );
     metrics.state_transition.process_block.observe(readSeconds(&timer));
 
-    metrics.state_transition.onPostState(post_state);
+    try metrics.state_transition.onPostState(post_state);
 
     // Verify state root
     if (opts.verify_state_root) {
         timer = try Timer.start();
         const post_state_root = try post_state.state.hashTreeRoot();
-
         try metrics.state_transition.state_hash_tree_root.observe(.{ .source = .compute_new_state_root }, readSeconds(&timer));
 
         const block_state_root = switch (block) {