Add test for exponential distribution serializer.
Add test for exponential distribution serializer.

Growth Factor: 2
Scale: 1

Expected frequency counts:
┌──────┬────────────────┬──────────┬─────┐
│Bucket│Width           │Samples   │Count│
├──────┼────────────────┼──────────┼─────┤
│0     │(−Inf,1)        │0         │1    │
│1     │[1,2)           │          │0    │
│2     │[2,4)           │          │0    │
│3     │[4,8)           │4         │1    │
│4     │[8,16)          │          │0    │
│5     │[16,32)         │          │0    │
│6     │[32,64)         │          │0    │
│7     │[64,128)        │          │0    │
│8     │[128,256)       │          │0    │
│9     │[256,512)       │          │0    │
│10    │[512,1024)      │          │0    │
│11    │[1024,2048)     │1024, 1536│2    │
│12    │[2048,4096)     │          │0    │
│13    │[4096,8192)     │          │0    │
│14    │[8192,16384)    │          │0    │
│15    │[16384,32768)   │          │0    │
│16    │[32768,65536)   │          │0    │
│17    │[65536,131072)  │          │0    │
│18    │[131072,262144) │          │0    │
│19    │[262144,524288) │          │0    │
│20    │[524288,1048576)│          │0    │
│21    │[1048576,+Inf)  │1048576   │1    │
└──────┴────────────────┴──────────┴─────┘

Counts: 5
Mean: (0+4+1024+1536+1048576) / 5 = 210228

BUG=b/191414589

Change-Id: I01a93e1169eef9ec341ba155bade14748ac9a3d5
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/3121705
Commit-Queue: Michal Kiedys <472a1e6216c9a44ed7bded64510d0626a6b6ca20@google.com>
Reviewed-by: Scott Lee <1dcb292530f82960316cb3d87cd67c3f212b79cb@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go