Fixes https://github.com/open-telemetry/opentelemetry-go/issues/7851
This adopts this proposal from @MrAlias:
https://github.com/open-telemetry/opentelemetry-go/issues/7851#issuecomment-3837399805
This adds a Set function to attrOpt, and changes attrOpt functions to
use a pointer receiver. Users can use this by checking if the attribute
option implements the `Settable` interface in `metric/x`. There are no
public API changes to the metric package, as this is still an
experiment. See the benchmark change and the interface documentation for
how it can be used to avoid calling metric.WithAttributeSet.
I updated the benchmark in
https://github.com/open-telemetry/opentelemetry-go/commit/448394b549375f4f6742201e199e0339d0b78524,
but didn't commit it here to avoid a dependency between the SDK and the
`metric/x` package.
As expected, this removes one allocation from the Dynamic case!
The results are:
```
│ main.txt │ resettable.txt │
│ sec/op │ sec/op vs base │
EndToEndCounterAdd/NoFilter/Attributes/1/Dynamic/WithAttributeSet-24 238.2n ± 5% 220.7n ± 9% -7.39% (p=0.002 n=6)
EndToEndCounterAdd/NoFilter/Attributes/5/Dynamic/WithAttributeSet-24 611.1n ± 4% 620.3n ± 2% ~ (p=0.394 n=6)
EndToEndCounterAdd/NoFilter/Attributes/10/Dynamic/WithAttributeSet-24 1.147µ ± 7% 1.171µ ± 5% ~ (p=0.258 n=6)
EndToEndCounterAdd/Filtered/Attributes/1/Dynamic/WithAttributeSet-24 363.8n ± 6% 363.5n ± 6% ~ (p=1.000 n=6)
EndToEndCounterAdd/Filtered/Attributes/5/Dynamic/WithAttributeSet-24 1.464µ ± 5% 1.475µ ± 10% ~ (p=0.727 n=6)
EndToEndCounterAdd/Filtered/Attributes/10/Dynamic/WithAttributeSet-24 2.924µ ± 7% 2.589µ ± 6% -11.47% (p=0.002 n=6)
│ main.txt │ resettable.txt │
│ B/op │ B/op vs base │
EndToEndCounterAdd/NoFilter/Attributes/1/Dynamic/WithAttributeSet-24 88.00 ± 0% 64.00 ± 0% -27.27% (p=0.002 n=6)
EndToEndCounterAdd/NoFilter/Attributes/5/Dynamic/WithAttributeSet-24 344.0 ± 0% 321.0 ± 0% -6.69% (p=0.002 n=6)
EndToEndCounterAdd/NoFilter/Attributes/10/Dynamic/WithAttributeSet-24 729.0 ± 0% 706.0 ± 0% -3.16% (p=0.002 n=6)
EndToEndCounterAdd/Filtered/Attributes/1/Dynamic/WithAttributeSet-24 152.0 ± 0% 128.0 ± 0% -15.79% (p=0.002 n=6)
EndToEndCounterAdd/Filtered/Attributes/5/Dynamic/WithAttributeSet-24 921.0 ± 0% 899.0 ± 0% -2.39% (p=0.002 n=6)
EndToEndCounterAdd/Filtered/Attributes/10/Dynamic/WithAttributeSet-24 2.026Ki ± 0% 2.006Ki ± 0% -1.01% (p=0.002 n=6)
│ main.txt │ resettable.txt │
│ allocs/op │ allocs/op vs base │
EndToEndCounterAdd/NoFilter/Attributes/1/Dynamic/WithAttributeSet-24 2.000 ± 0% 1.000 ± 0% -50.00% (p=0.002 n=6)
EndToEndCounterAdd/NoFilter/Attributes/5/Dynamic/WithAttributeSet-24 2.000 ± 0% 1.000 ± 0% -50.00% (p=0.002 n=6)
EndToEndCounterAdd/NoFilter/Attributes/10/Dynamic/WithAttributeSet-24 2.000 ± 0% 1.000 ± 0% -50.00% (p=0.002 n=6)
EndToEndCounterAdd/Filtered/Attributes/1/Dynamic/WithAttributeSet-24 3.000 ± 0% 2.000 ± 0% -33.33% (p=0.002 n=6)
EndToEndCounterAdd/Filtered/Attributes/5/Dynamic/WithAttributeSet-24 4.000 ± 0% 3.000 ± 0% -25.00% (p=0.002 n=6)
EndToEndCounterAdd/Filtered/Attributes/10/Dynamic/WithAttributeSet-24 4.000 ± 0% 3.000 ± 0% -25.00% (p=0.002 n=6)
```
---------
Co-authored-by: Tyler Yahn <MrAlias@users.noreply.github.com>
David Ashpole
Tyler Yahn
Matthieu MOREL
Robert Pająk