feat(fork_choice): integrate metrics into fork choice module

Wire up the metrics dependency (build.zig, zbuild.zon), export the
metrics submodule from root.zig, and instrument computeDeltas in
fork_choice.zig with timing and counter observations.

Author: GrapeBaBa

build.zig

@@ -1217,6 +1217,7 @@ pub fn build(b: *std.Build) void {
     module_fork_choice.addImport("fork_types", module_fork_types);
     module_fork_choice.addImport("hex", module_hex);
     module_fork_choice.addImport("constants", module_constants);
+    module_fork_choice.addImport("metrics", dep_metrics.module("metrics"));
 
     module_download_era_files.addImport("download_era_options", options_module_download_era_options);
 

src/fork_choice/fork_choice.zig

@@ -42,6 +42,8 @@ const compute_deltas = @import("compute_deltas.zig");
 const computeDeltas = compute_deltas.computeDeltas;
 const DeltasCache = compute_deltas.DeltasCache;
 
+const metrics = @import("metrics.zig");
+
 const store = @import("store.zig");
 pub const ForkChoiceStore = store.ForkChoiceStore;
 pub const Checkpoint = store.Checkpoint;
@@ -942,6 +944,8 @@ pub const ForkChoice = struct {
         const old_balances = self.balances.get().items;
         const new_balances = self.fc_store.justified.balances.get().items;
 
+        var compute_deltas_timer = try std.time.Timer.start();
+
         const vote_fields = self.votes.fields();
         const result = try computeDeltas(
             allocator,
@@ -954,6 +958,21 @@ pub const ForkChoice = struct {
             &self.fc_store.equivocating_indices,
         );
 
+        metrics.fork_choice_metrics.compute_deltas_duration.observe(metrics.readSeconds(&compute_deltas_timer));
+        metrics.fork_choice_metrics.compute_deltas_deltas_count.set(@intCast(result.deltas.len));
+        metrics.fork_choice_metrics.compute_deltas_equivocating_validators.set(result.equivocating_validators);
+        metrics.fork_choice_metrics.compute_deltas_old_inactive_validators.set(result.old_inactive_validators);
+        metrics.fork_choice_metrics.compute_deltas_new_inactive_validators.set(result.new_inactive_validators);
+        metrics.fork_choice_metrics.compute_deltas_unchanged_vote_validators.set(result.unchanged_vote_validators);
+        metrics.fork_choice_metrics.compute_deltas_new_vote_validators.set(result.new_vote_validators);
+
+        // Count zero deltas.
+        var zero_count: u64 = 0;
+        for (result.deltas) |d| {
+            if (d == 0) zero_count += 1;
+        }
+        metrics.fork_choice_metrics.compute_deltas_zero_deltas_count.set(zero_count);
+
         self.balances.release();
         self.balances = self.fc_store.justified.balances.acquire();
 

src/fork_choice/root.zig

@@ -6,6 +6,7 @@ pub const compute_deltas = @import("compute_deltas.zig");
 pub const proto_array = @import("proto_array.zig");
 pub const store = @import("store.zig");
 pub const fork_choice = @import("fork_choice.zig");
+pub const metrics = @import("metrics.zig");
 
 pub const ProtoBlock = proto_array.ProtoBlock;
 pub const ProtoNode = proto_array.ProtoNode;

zbuild.zon

@@ -145,6 +145,7 @@
                 .fork_types,
                 .hex,
                 .constants,
+                .metrics,
             },
         },
     },