refactor(benchmark-results): emit numeric values instead of strings

[ilteoood]

The committed benchmark-results.json and the file produced by
benchmark-compare.js had requests, latency, and throughput
encoded as strings (e.g. "54500.8") and "N/A" for missing
values. Numeric JSON consumers can't operate on those directly.

This change emits them as numbers (using Math.round with the same
decimal precision as before) and null when a metric is absent,
while keeping the rounding behaviour identical for present values.

-  const formatThroughput = throughput => throughput ? (throughput / 1024 / 1024).toFixed(2) : 'N/A'
+  const round = (value, decimals) => value ? Math.round(value * 10 ** decimals) / 10 ** decimals : null
...
-        requests: requests ? requests.toFixed(1) : 'N/A',
-        latency: latency ? latency.toFixed(2) : 'N/A',
-        throughput: formatThroughput(throughput)
+        requests: round(requests, 1),
+        latency: round(latency, 2),
+        throughput: round(throughput / 1024 / 1024, 2)

benchmark-results.json is regenerated to match so the checked-in
file stays consistent with the new format.