Speed up timestamp generation when logging (#9933)

* Speed up timestamp generation when logging

* Align to review comments.

* Correct a typo

substrate/client/tracing/benches/bench.rs

@@ -0,0 +1,49 @@
+// This file is part of Substrate.
+
+// Copyright (C) 2021 Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use criterion::{criterion_group, criterion_main, Criterion};
+use tracing_subscriber::fmt::time::{ChronoLocal, FormatTime};
+
+fn bench_fast_local_time(c: &mut Criterion) {
+	c.bench_function("fast_local_time", |b| {
+		let mut buffer = String::new();
+		let t = sc_tracing::logging::FastLocalTime { with_fractional: true };
+		b.iter(|| {
+			buffer.clear();
+			t.format_time(&mut buffer).unwrap();
+		})
+	});
+}
+
+// This is here just as a point of comparison.
+fn bench_chrono_local(c: &mut Criterion) {
+	c.bench_function("chrono_local", |b| {
+		let mut buffer = String::new();
+		let t = ChronoLocal::with_format("%Y-%m-%d %H:%M:%S%.3f".to_string());
+		b.iter(|| {
+			buffer.clear();
+			t.format_time(&mut buffer).unwrap();
+		})
+	});
+}
+
+criterion_group! {
+	name = benches;
+	config = Criterion::default();
+	targets = bench_fast_local_time, bench_chrono_local
+}
+criterion_main!(benches);