perf: add benchmark for the get_metrics endpoint. (#238)

Adds a benchmark to count how many instructions it takes to run the
`get_metrics` endpoint when there is a large number of unstable blocks.

The output of `benchmarks/run.sh` is currently:

```
(1_519_453_070 : nat64) <-- benchmark for `insert_300_blocks`
(163_059_419 : nat64)   <-- benchmark for `get_metrics`
```

benchmarks/drun.txt

@@ -1,3 +1,4 @@
 create
 install rwlgt-iiaaa-aaaaa-aaaaa-cai ../target/wasm32-unknown-unknown/release/benchmarks.wasm.gz ""
 query rwlgt-iiaaa-aaaaa-aaaaa-cai insert_300_blocks "DIDL\x00\x00"
+query rwlgt-iiaaa-aaaaa-aaaaa-cai get_metrics "DIDL\x00\x00"

benchmarks/run.sh

@@ -12,4 +12,4 @@ bash ../scripts/build-canister.sh benchmarks
 drun ./drun.txt --instruction-limit 99999999999999 \
     | awk '{ print $3 }' \
     | grep "44.*" -o \
-    | xargs didc decode
+    | xargs -L 1 didc decode

benchmarks/src/main.rs

@@ -11,12 +11,6 @@ thread_local! {
 
 #[init]
 fn init() {
-    ic_btc_canister::init(Config {
-        network: Network::Testnet,
-        stability_threshold: 144,
-        ..Config::default()
-    });
-
     // Load the testnet blocks.
     TESTNET_BLOCKS.with(|blocks| {
         blocks.replace(
@@ -35,6 +29,12 @@ fn init() {
 // Benchmarks inserting the first 300 blocks of the Bitcoin testnet.
 #[query]
 fn insert_300_blocks() -> u64 {
+    ic_btc_canister::init(Config {
+        network: Network::Testnet,
+        stability_threshold: 144,
+        ..Config::default()
+    });
+
     count_instructions(|| {
         with_state_mut(|s| {
             for i in 0..300 {
@@ -48,6 +48,30 @@ fn insert_300_blocks() -> u64 {
     })
 }
 
+// Benchmarks gettings the metrics when there are many unstable blocks..
+#[query]
+fn get_metrics() -> u64 {
+    ic_btc_canister::init(Config {
+        network: Network::Testnet,
+        stability_threshold: 3000,
+        ..Config::default()
+    });
+
+    with_state_mut(|s| {
+        for i in 0..3000 {
+            ic_btc_canister::state::insert_block(
+                s,
+                TESTNET_BLOCKS.with(|b| b.borrow()[i as usize].clone()),
+            )
+            .unwrap();
+        }
+    });
+
+    count_instructions(|| {
+        ic_btc_canister::get_metrics();
+    })
+}
+
 // Returns the number of instructions consumed by the given function.
 fn count_instructions<R>(f: impl FnOnce() -> R) -> u64 {
     let start = ic_cdk::api::performance_counter(0);