This attempts to improve the quality of the on-CPU profiles stackprof provides. Rather than weighing samples by their timestamp deltas, which, in our opinion, are only valid in wall-clock mode, this weighs callchains by:
```
S = number of samples
P = sample period in nanoseconds
W = S * P
```
The difference after this change is quite substantial, specially in profiles that previously were showing up with heavy IO frames:
* Total profile weight is almost down by 90%, which actually makes sense for an on-CPU profile if the app is relatively idle
* Certain callchains that blocked in syscalls / IO are now much lower weight. This was what I was expecting to find.
Here is an example of the latter point.
In delta mode, we see an io select taking a long time, it is a significant portion of the profile:
<img width="1100" alt="236936508-709bee01-d616-4246-ba74-ab004331dcd3" src="https://github.com/dalehamel/speedscope/assets/4398256/39140f1e-50a9-4f33-8a61-ec98b6273fd4">
But in period scaling mode, it is only a couple of sample periods ultimately:
<img width="206" alt="236936693-9d44304e-a1c2-4906-b3c8-50e19e6f9f27" src="https://github.com/dalehamel/speedscope/assets/4398256/7d19077f-ef25-4d79-980b-cfa1775d928d">
This PR attempts to support stackprof's object mode which tracks the number of allocated objects. This differs from the other modes (cpu and wall) by taking samples every time a Ruby object is allocated using Ruby's [`NEWOBJ` tracepoint](df24b85953/ext/stackprof/stackprof.c (L198-L199)).
When importing an object mode profile into speedscope today it still works but what you see is a profile using time units. The profile will only have samples for when an object was allocated which means even if time is reported, the profile is not really meaningful when looking at time.
To address this I've done three things when `mode` is `object`:
+ adjusted the total size of the `StackListProfileBuilder` to use the number of samples (since each sample is one allocation)
+ adjusted the weight of each sample to be `nSamples` (which I believe is always `1` but I'm not positive)
+ do not set the value formatter to a time formatter
Here's what it looks like before and after my changes (note the units and weight of samples):
wall (before) | object (before) | object (after)
-- | -- | --
<img width="1624" alt="Screen Shot 2022-05-11 at 4 51 31 PM" src="https://user-images.githubusercontent.com/898172/167945635-2401ca73-4de7-4559-b884-cf8947ca9738.png"> | <img width="1624" alt="Screen Shot 2022-05-11 at 4 51 34 PM" src="https://user-images.githubusercontent.com/898172/167945641-ef302a60-730b-4afd-8e44-5f02e54b3cb7.png"> | <img width="1624" alt="Screen Shot 2022-05-11 at 4 51 42 PM" src="https://user-images.githubusercontent.com/898172/167945643-5611b267-f8b2-4227-a2bf-7145c4030aa2.png">
<details>
<summary>Test code</summary>
```ruby
require 'stackprof'
require 'json'
def do_test
5.times do
make_a_word
end
end
def make_a_word
('a'..'z').to_a.shuffle.map(&:upcase).join
end
StackProf.start(mode: :object, interval: 1, raw: true)
do_test
StackProf.stop
File.write('tmp/object_profile.json', JSON.generate(StackProf.results))
StackProf.start(mode: :wall, interval: 1, raw: true)
do_test
StackProf.stop
File.write('tmp/wall_profile.json', JSON.generate(StackProf.results))
```
</details>
