Implement custom OpenTelemetry sampler to filter uninstrumented traces (#8647)

Co-Authored-By: Jimmy Chen <jchen.tc@gmail.com>

common/tracing_samplers/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "tracing_samplers"
+version = "0.1.0"
+edition = { workspace = true }
+
+[dependencies]
+opentelemetry = { workspace = true }
+opentelemetry_sdk = { workspace = true }

common/tracing_samplers/src/lib.rs

@@ -0,0 +1,78 @@
+//! OpenTelemetry samplers for filtering traces.
+
+use opentelemetry::Context;
+use opentelemetry::trace::{Link, SamplingDecision, SamplingResult, SpanKind, TraceState};
+use opentelemetry_sdk::trace::ShouldSample;
+
+/// A sampler that only samples spans whose names start with a given prefix.
+///
+/// This sampler is designed to be used with `Sampler::ParentBased`, which will:
+/// - Use this sampler's decision for root spans (no parent)
+/// - Automatically inherit the parent's sampling decision for child spans
+///
+/// This ensures that only traces starting from known instrumented code paths are exported,
+/// reducing noise from partially instrumented code paths.
+#[derive(Debug, Clone)]
+pub struct PrefixBasedSampler {
+    prefix: &'static str,
+}
+
+impl PrefixBasedSampler {
+    pub fn new(prefix: &'static str) -> Self {
+        Self { prefix }
+    }
+}
+
+impl ShouldSample for PrefixBasedSampler {
+    fn should_sample(
+        &self,
+        _parent_context: Option<&Context>,
+        _trace_id: opentelemetry::trace::TraceId,
+        name: &str,
+        _span_kind: &SpanKind,
+        _attributes: &[opentelemetry::KeyValue],
+        _links: &[Link],
+    ) -> SamplingResult {
+        if name.starts_with(self.prefix) {
+            SamplingResult {
+                decision: SamplingDecision::RecordAndSample,
+                attributes: Vec::new(),
+                trace_state: TraceState::default(),
+            }
+        } else {
+            SamplingResult {
+                decision: SamplingDecision::Drop,
+                attributes: Vec::new(),
+                trace_state: TraceState::default(),
+            }
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use opentelemetry::trace::TraceId;
+
+    #[test]
+    fn prefix_based_sampler_filters_by_prefix() {
+        let sampler = PrefixBasedSampler::new("test_");
+        let trace_id = TraceId::from_hex("0123456789abcdef0123456789abcdef").unwrap();
+
+        // Spans with prefix should be sampled
+        let result = sampler.should_sample(
+            None,
+            trace_id,
+            "test_my_span",
+            &SpanKind::Internal,
+            &[],
+            &[],
+        );
+        assert!(matches!(result.decision, SamplingDecision::RecordAndSample));
+
+        // Spans without prefix should be dropped
+        let result =
+            sampler.should_sample(None, trace_id, "other_span", &SpanKind::Internal, &[], &[]);
+        assert!(matches!(result.decision, SamplingDecision::Drop));
+    }
+}