fix(bench): explicit timers don't force high precision measurements (#20272)

Disables `BenchContext::start()` and `BenchContext::end()` for low
precision benchmarks (less than 0.01s per iteration). Prints a warning
when they are used in such benchmarks, suggesting to remove them.

```ts
Deno.bench("noop", { group: "noops" }, () => {});

Deno.bench("noop with start/end", { group: "noops" }, (b) => {
  b.start();
  b.end();
});
```

Before:
```
cpu: 12th Gen Intel(R) Core(TM) i9-12900K
runtime: deno 1.36.2 (x86_64-unknown-linux-gnu)

file:///home/nayeem/projects/deno/temp3.ts
benchmark                    time (avg)        iter/s             (min … max)       p75       p99      p995
----------------------------------------------------------------------------- -----------------------------


noop                          2.63 ns/iter 380,674,131.4    (2.45 ns … 27.78 ns)   2.55 ns   4.03 ns   5.33 ns
noop with start and end     302.47 ns/iter   3,306,146.0     (200 ns … 151.2 µs)    300 ns    400 ns    400 ns

summary
  noop
   115.14x faster than noop with start and end
```

After:
```
cpu: 12th Gen Intel(R) Core(TM) i9-12900K
runtime: deno 1.36.1 (x86_64-unknown-linux-gnu)

file:///home/nayeem/projects/deno/temp3.ts
benchmark                    time (avg)        iter/s             (min … max)       p75       p99      p995
----------------------------------------------------------------------------- -----------------------------


noop                          3.01 ns/iter 332,565,561.7    (2.73 ns … 29.54 ns)   2.93 ns   5.29 ns   7.45 ns
noop with start and end       7.73 ns/iter 129,291,091.5    (6.61 ns … 46.76 ns)   7.87 ns  13.12 ns  15.32 ns
Warning start() and end() calls in "noop with start and end" are ignored because it averages less than 0.01s per iteration. Remove them for better results.

summary
  noop
   2.57x faster than noop with start and end
```

1 files changed, 15 insertions, 5 deletions

diff --git a/cli/js/40_testing.js b/cli/js/40_testing.js
index 21417db23..afa9a6552 100644
--- a/cli/js/40_testing.js
+++ b/cli/js/40_testing.js
@@ -849,7 +849,7 @@ function compareMeasurements(a, b) {
   return 0;
 }
 
-function benchStats(n, highPrecision, avg, min, max, all) {
+function benchStats(n, highPrecision, usedExplicitTimers, avg, min, max, all) {
   return {
     n,
     min,
@@ -859,6 +859,8 @@ function benchStats(n, highPrecision, avg, min, max, all) {
     p995: all[MathCeil(n * (99.5 / 100)) - 1],
     p999: all[MathCeil(n * (99.9 / 100)) - 1],
     avg: !highPrecision ? (avg / n) : MathCeil(avg / n),
+    highPrecision,
+    usedExplicitTimers,
   };
 }
 
@@ -926,7 +928,7 @@ async function benchMeasure(timeBudget, fn, async, context) {
   wavg /= c;
 
   // measure step
-  if (wavg > lowPrecisionThresholdInNs || usedExplicitTimers) {
+  if (wavg > lowPrecisionThresholdInNs) {
     let iterations = 10;
     let budget = timeBudget * 1e6;
 
@@ -978,6 +980,8 @@ async function benchMeasure(timeBudget, fn, async, context) {
       }
     }
   } else {
+    context.start = function start() {};
+    context.end = function end() {};
     let iterations = 10;
     let budget = timeBudget * 1e6;
 
@@ -986,8 +990,6 @@ async function benchMeasure(timeBudget, fn, async, context) {
         const t1 = benchNow();
         for (let c = 0; c < lowPrecisionThresholdInNs; c++) {
           fn(context);
-          currentBenchUserExplicitStart = null;
-          currentBenchUserExplicitEnd = null;
         }
         const iterationTime = (benchNow() - t1) / lowPrecisionThresholdInNs;
 
@@ -1019,7 +1021,15 @@ async function benchMeasure(timeBudget, fn, async, context) {
   }
 
   all.sort(compareMeasurements);
-  return benchStats(n, wavg > lowPrecisionThresholdInNs, avg, min, max, all);
+  return benchStats(
+    n,
+    wavg > lowPrecisionThresholdInNs,
+    usedExplicitTimers,
+    avg,
+    min,
+    max,
+    all,
+  );
 }
 
 /** @param desc {BenchDescription} */