Two new benchmarks here-in.
Benchmark 1: `bm_picker`
------
Measures various load balancing policies pick performance. For now we cover `pick_first` and `weighted_round_robin` at 1, 10, 100, 1000, 10000, and 100000 backends.
Today's output:
```
------------------------------------------------------------------------------
Benchmark Time CPU Iterations
------------------------------------------------------------------------------
BM_Pick/pick_first/1 20.4 ns 20.4 ns 68285
BM_Pick/pick_first/10 20.6 ns 20.6 ns 68274
BM_Pick/pick_first/100 20.5 ns 20.5 ns 67817
BM_Pick/pick_first/1000 20.6 ns 20.6 ns 67347
BM_Pick/pick_first/10000 20.7 ns 20.7 ns 67317
BM_Pick/pick_first/100000 20.9 ns 20.9 ns 67385
BM_Pick/weighted_round_robin/1 54.7 ns 54.7 ns 26641
BM_Pick/weighted_round_robin/10 54.2 ns 54.2 ns 25828
BM_Pick/weighted_round_robin/100 55.2 ns 55.2 ns 26210
BM_Pick/weighted_round_robin/1000 54.1 ns 54.1 ns 25678
BM_Pick/weighted_round_robin/10000 77.3 ns 76.6 ns 15776
BM_Pick/weighted_round_robin/100000 148 ns 148 ns 9882
```
Benchmark 2: `bm_load_balanced_call_destination`
-----
This benchmark measures call performance when a call spine passes through a `LoadBalancedCallDestination`, and with `BM_LoadBalancedCallDestination` also the construction/destruction cost of this object.
We do not consider picker performance in this benchmark as it's separately covered by `bm_picker` above.
Today's output:
```
-----------------------------------------------------------------------------------------------------------------------------------------
Benchmark Time CPU Iterations
-----------------------------------------------------------------------------------------------------------------------------------------
BM_UnaryWithSpawnPerEnd<UnstartedCallDestinationFixture<LoadBalancedCallDestinationTraits>> 1255 ns 1255 ns 1076
BM_UnaryWithSpawnPerOp<UnstartedCallDestinationFixture<LoadBalancedCallDestinationTraits>> 1459 ns 1459 ns 939
BM_ClientToServerStreaming<UnstartedCallDestinationFixture<LoadBalancedCallDestinationTraits>> 209 ns 209 ns 6775
BM_LoadBalancedCallDestination 92.8 ns 92.8 ns 15063
```
Notes
------
There's some duplicated code between the benchmarks & tests -- this is ok -- as the tests evolve we'll likely want to add more checks to the fixtures, whereas as the benchmarks evolve we may well want to optimize the fixtures so that performance of the systems under test dominate more. That is, the duplicated code is expected to have different evolutionary tracks.
Closes#37052
COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/37052 from ctiller:moar-benchy 30c7072d87
PiperOrigin-RevId: 658181731
Craig Tiller