Differential flamegraphs are incorrectly scaled across sampling rates

Differential flamegraphs compare raw sample counts after scaling the baseline by sampled wall time. This makes identical profiles look very different when the sampling rates differ.

For example, using a 1 kHz baseline and profiling the same workload at 100 Hz reports around -90% for unchanged leaf functions. Using the same rate for both profiles reports approximately 0%.

We should normalize the sample counts consistently and use the sampling interval only when converting samples to time for display.

Linked PRs

• gh-154082

首先使用 1 kHz 和 100 Hz 的 profile 重现 issue 中描述的 differential flamegraph 比较，然后追踪 sample 计数在哪里进行缩放，以及 sampling interval 在哪里将 sample 转换为显示时间。当不同 sampling rate 下的相同 profile 比较结果约为 0% 且显示时间转换仍然正确时，即可视为完成。