feat: Add sections for new subnet_stats endpoint

spec/_attachments/ic.did

@@ -342,6 +342,14 @@ type node_metrics_history_result = vec record {
     node_metrics : vec node_metrics;
 };
 
+type subnet_metrics_args = record {
+    subnet_id : principal;
+};
+
+type subnet_metrics_result = record {
+    replica_version : text;
+};
+
 type provisional_create_canister_with_cycles_args = record {
     amount : opt nat;
     settings : opt canister_settings;
@@ -444,6 +452,9 @@ service ic : {
     // metrics interface
     node_metrics_history : (node_metrics_history_args) -> (node_metrics_history_result);
 
+    // subnet metrics
+    subnet_metrics : (subnet_metrics_args) -> (subnet_metrics_result);
+
     // provisional interfaces for the pre-ledger world
     provisional_create_canister_with_cycles : (provisional_create_canister_with_cycles_args) -> (provisional_create_canister_with_cycles_result);
     provisional_top_up_canister : (provisional_top_up_canister_args) -> ();

spec/index.md

@@ -2548,6 +2548,14 @@ A single metric entry is a record with the following fields:
 
 - `num_block_failures_total` (`nat64`): the number of failed block proposals by this node.
 
+### IC method `subnet_metrics` {#ic-subnet-metrics}
+
+This method can only be called by canisters, i.e., it cannot be called by external users via ingress messages.
+
+Given a subnet ID as input, this method returns a record `subnet_metrics` containing metadata about that subnet.
+
+Currently, the only field returned is the `replica_version` (`text`) of the targeted subnet.
+
 ### IC method `take_canister_snapshot` {#ic-take_canister_snapshot}
 
 This method can be called by canisters as well as by external users via ingress messages.
@@ -5439,6 +5447,33 @@ S with
 
 ```
 
+#### IC Management Canister: Subnet Metrics
+
+The management canister returns subnet metadata given a subnet ID.
+
+Conditions
+
+```html
+S.messages = Older_messages · CallMessage M · Younger_messages 
+(M.queue = Unordered) or (∀ CallMessage M' | FuncMessage M' ∈ Older_messages. M'.queue ≠ M.queue) 
+M.callee = ic_principal 
+M.method_name = 'subnet_metrics' 
+R = <implementation-specific> 
+```
+
+State after
+
+```html
+S with 
+    messages = Older_messages · Younger_messages · 
+      ResponseMessage { 
+        origin = M.origin 
+        response = Reply (candid(R)) 
+        refunded_cycles = M.transferred_cycles 
+      }
+```
+
+
 #### IC Management Canister: Canister creation with cycles
 
 This is a variant of `create_canister`, which sets the initial cycle balance based on the `amount` argument.