crtime: add MonoFromTime method

This is useful when we have a `time.Time` that we can't change to use
`Mono`, for example `ctx.Deadline()`.

crtime/monotonic.go

@@ -40,6 +40,12 @@ func (m Mono) Elapsed() time.Duration {
 	return time.Duration(NowMono() - m)
 }
 
+// MonoFromTime converts a time.Time to a Mono value. If the time has a
+// monotonic component, it is used.
+func MonoFromTime(t time.Time) Mono {
+	return Mono(t.Sub(startTime))
+}
+
 // We use startTime as a reference point against which we can call
 // time.Since(). This solution is suggested by the Go runtime code:
 // https://github.com/golang/go/blob/889abb17e125bb0f5d8de61bb80ef15fbe2a130d/src/runtime/time_nofake.go#L19

crtime/monotonic_test.go

@@ -0,0 +1,34 @@
+// Copyright 2024 The Cockroach Authors.
+//
+// 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.
+
+package crtime
+
+import (
+	"testing"
+	"time"
+)
+
+func TestMono(t *testing.T) {
+	a := NowMono()
+	time.Sleep(10 * time.Millisecond)
+	b := NowMono()
+	if delta := b.Sub(a); delta < 9*time.Millisecond {
+		t.Errorf("expected 10+ms, got %s", delta)
+	}
+	c := MonoFromTime(time.Now())
+	d := NowMono()
+	if c < b || c > d {
+		t.Errorf("expected %d <= %d <= %d", b, c, d)
+	}
+}