See #7441

```
### Errors in sdk/metric/exemplar/fixed_size_reservoir.go

* [403] <https://dl.acm.org/doi/10.1145/198429.198435> | Rejected status code (this depends on your "accept" configuration): Forbidden
```

~Depends on #7441, #7443~

This improves the concurrent performance of the fixed size reservoir's
Offer function by 4x (i.e. 75% reduction). This improves the performance
of Measure() for fixed-size reservoirs by 60% overall.

Accomplish this by:

* using a single atomic for count and next. This assumes that both can
fit in a uint32.
* only use a lock to guard changing `w` and `next` together.

Offer benchmarks:
```
                           │   main.txt   │           fixedsize.txt            │
                           │    sec/op    │   sec/op     vs base               │
FixedSizeReservoirOffer-24   185.25n ± 4%   45.58n ± 1%  -75.40% (p=0.002 n=6)
```

Measure benchmarks:
```
                                                                          │   main.txt   │            fixedsize.txt            │
                                                                          │    sec/op    │    sec/op     vs base               │
SyncMeasure/NoView/ExemplarsEnabled/Int64Counter/Attributes/0-24            175.45n ± 6%   67.01n ±  9%  -61.81% (p=0.002 n=6)
SyncMeasure/NoView/ExemplarsEnabled/Int64Counter/Attributes/1-24            170.25n ± 1%   69.82n ±  6%  -58.99% (p=0.002 n=6)
SyncMeasure/NoView/ExemplarsEnabled/Int64Counter/Attributes/10-24           167.40n ± 2%   64.52n ± 10%  -61.46% (p=0.002 n=6)
SyncMeasure/NoView/ExemplarsEnabled/Float64Counter/Attributes/0-24          173.55n ± 0%   69.17n ± 12%  -60.14% (p=0.002 n=6)
SyncMeasure/NoView/ExemplarsEnabled/Float64Counter/Attributes/1-24          169.50n ± 1%   68.55n ±  5%  -59.56% (p=0.002 n=6)
SyncMeasure/NoView/ExemplarsEnabled/Float64Counter/Attributes/10-24         166.95n ± 1%   65.82n ±  6%  -60.58% (p=0.002 n=6)
SyncMeasure/NoView/ExemplarsEnabled/Int64UpDownCounter/Attributes/0-24      168.85n ± 1%   67.99n ± 11%  -59.73% (p=0.002 n=6)
SyncMeasure/NoView/ExemplarsEnabled/Int64UpDownCounter/Attributes/1-24      173.50n ± 1%   66.69n ±  2%  -61.56% (p=0.002 n=6)
SyncMeasure/NoView/ExemplarsEnabled/Int64UpDownCounter/Attributes/10-24     171.30n ± 5%   67.73n ±  8%  -60.46% (p=0.002 n=6)
SyncMeasure/NoView/ExemplarsEnabled/Float64UpDownCounter/Attributes/0-24    168.90n ± 2%   67.69n ±  9%  -59.92% (p=0.002 n=6)
SyncMeasure/NoView/ExemplarsEnabled/Float64UpDownCounter/Attributes/1-24    173.35n ± 2%   68.25n ±  9%  -60.63% (p=0.002 n=6)
SyncMeasure/NoView/ExemplarsEnabled/Float64UpDownCounter/Attributes/10-24   172.95n ± 2%   70.90n ±  7%  -59.01% (p=0.002 n=6)
geomean                                                                      171.0n        67.83n        -60.33%
```

---------

Co-authored-by: Tyler Yahn <MrAlias@users.noreply.github.com>
Co-authored-by: Robert Pająk <pellared@hotmail.com>