Move to SmallRng from ThreadRng

SmallRng provides 0-6% improvement in Traces.

Relates #808

Author: hdost

opentelemetry-sdk/Cargo.toml

@@ -21,7 +21,7 @@ futures-util = { version = "0.3.17", default-features = false, features = ["std"
 once_cell = "1.10"
 ordered-float = "4.0"
 percent-encoding = { version = "2.0", optional = true }
-rand = { version = "0.8", default-features = false, features = ["std", "std_rng"], optional = true }
+rand = { version = "0.8", default-features = false, features = ["std", "std_rng","small_rng"], optional = true }
 glob = {version = "0.3.1", optional =true}
 serde = { version = "1.0", features = ["derive", "rc"], optional = true }
 serde_json = { version = "1", optional = true }
@@ -68,6 +68,7 @@ harness = false
 [[bench]]
 name = "trace"
 harness = false
+required-features = ["testing"]
 
 [[bench]]
 name = "batch_span_processor"

opentelemetry-sdk/src/trace/id_generator/mod.rs

@@ -2,7 +2,7 @@
 pub(super) mod aws;
 
 use opentelemetry::trace::{SpanId, TraceId};
-use rand::{rngs, Rng};
+use rand::{rngs, Rng, SeedableRng};
 use std::cell::RefCell;
 use std::fmt;
 
@@ -35,5 +35,5 @@ impl IdGenerator for RandomIdGenerator {
 
 thread_local! {
     /// Store random number generator for each thread
-    static CURRENT_RNG: RefCell<rngs::ThreadRng> = RefCell::new(rngs::ThreadRng::default());
+    static CURRENT_RNG: RefCell<rngs::SmallRng> = RefCell::new(rngs::SmallRng::from_entropy());
 }