feat(payload): adopt Probability for ValueConf::float_probability

Change `ValueConf::float_probability` from `f32` to the `Probability` alias
of `BoundedProbability<{ f32::to_bits(0.0) }>`. The `try_from` impl enforces
the finite + `[0.0, 1.0]` invariant at deserialize time; `ValueConf::new`
now takes the validated type directly. (`ValueConf::valid` already had no
finite/range check on this field, so nothing is removed there.)

The new type threads through `NumValueGenerator`'s `Constant` and `Uniform`
variants. At the two comparison sites in `<NumValueGenerator as Generator>::
generate`, `.get()` extracts the inner `f32` so RNG sequences and bit-exact
output are preserved.

Verified: `cargo test -p lading-payload` (250 pass). `cargo bench -p
lading-payload --bench dogstatsd --baseline pr1` shows ±2-6% swings across
all throughput sizes; the unrelated `cache_*` benches in `--bench block`
show similar magnitude run-to-run variance, so the delta is noise rather
than a real effect.

Author: goxberry

lading_payload/src/dogstatsd.rs

@@ -113,15 +113,15 @@ impl Default for MetricWeights {
 #[serde(deny_unknown_fields)]
 #[cfg_attr(feature = "arbitrary", derive(arbitrary::Arbitrary))]
 pub struct ValueConf {
-    /// Odds out of 256 that the value will be a float and not an integer.
-    float_probability: f32,
+    /// Probability that the value will be a float and not an integer.
+    float_probability: Probability,
     range: ConfRange<i64>,
 }
 
 impl ValueConf {
     /// Create a new instance of `ValueConf` according to the args
     #[must_use]
-    pub fn new(float_probability: f32, range: ConfRange<i64>) -> Self {
+    pub fn new(float_probability: Probability, range: ConfRange<i64>) -> Self {
         Self {
             float_probability,
             range,
@@ -136,7 +136,7 @@ impl ValueConf {
 impl Default for ValueConf {
     fn default() -> Self {
         Self {
-            float_probability: 0.5, // 50%
+            float_probability: Probability::try_new(0.5).expect("0.5 is in [0.0, 1.0]"),
             range: ConfRange::Inclusive {
                 min: i64::MIN,
                 max: i64::MAX,

lading_payload/src/dogstatsd/common.rs

@@ -6,7 +6,7 @@ use rand::{
     prelude::Distribution,
 };
 
-use crate::{Error, Generator};
+use crate::{Error, Generator, common::config::Probability};
 
 use super::{ConfRange, ValueConf};
 
@@ -21,12 +21,12 @@ pub enum NumValue {
 #[derive(Clone, Debug)]
 pub(crate) enum NumValueGenerator {
     Constant {
-        float_probability: f32,
+        float_probability: Probability,
         int: i64,
         float: f64,
     },
     Uniform {
-        float_probability: f32,
+        float_probability: Probability,
         int_distr: Uniform<i64>,
         float_distr: Uniform<f64>,
     },
@@ -67,7 +67,7 @@ impl<'a> Generator<'a> for NumValueGenerator {
                 int,
                 float,
             } => {
-                if prob < *float_probability {
+                if prob < float_probability.get() {
                     Ok(NumValue::Float(*float))
                 } else {
                     Ok(NumValue::Int(*int))
@@ -78,7 +78,7 @@ impl<'a> Generator<'a> for NumValueGenerator {
                 int_distr,
                 float_distr,
             } => {
-                if prob < *float_probability {
+                if prob < float_probability.get() {
                     Ok(NumValue::Float(float_distr.sample(rng)))
                 } else {
                     Ok(NumValue::Int(int_distr.sample(rng)))