Honor sampler environment variable if set (#724)

* Honor sampler envar if set

* Add entry to changelog

CHANGELOG.md

@@ -35,6 +35,8 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - Update `go.opentelemetry.io/contrib*` dependencies from
   [`v1.4.0`/`v0.30.0`][contrib-v1.4.0] to [`v1.6.0`/`v0.31.0`][contrib-v1.6.0].
   (#720)
+- The `OTEL_TRACES_SAMPLER` environment variable is now honored instead of only
+  defaulting to an always-on sampler. (#724)
 
 ### Fixed
 

distro/otel.go

@@ -38,6 +38,8 @@ import (
 
 var distroVerAttr = attribute.String("splunk.distro.version", splunkotel.Version())
 
+const tracesSamplerKey = "OTEL_TRACES_SAMPLER"
+
 const noServiceWarn = `service.name attribute is not set. Your service is unnamed and might be difficult to identify. Set your service name using the OTEL_SERVICE_NAME environment variable. For example, OTEL_SERVICE_NAME="<YOUR_SERVICE_NAME_HERE>")`
 
 // SDK contains all OpenTelemetry SDK state and provides access to this state.
@@ -100,11 +102,15 @@ func Run(opts ...Option) (SDK, error) {
 		c.Logger.Info(noServiceWarn)
 	}
 
-	traceProvider := trace.NewTracerProvider(
+	o := []trace.TracerProviderOption{
 		trace.WithResource(res),
-		trace.WithSampler(trace.AlwaysSample()),
 		trace.WithSpanProcessor(trace.NewBatchSpanProcessor(exp)),
-	)
+	}
+	if _, ok := os.LookupEnv(tracesSamplerKey); !ok {
+		o = append(o, trace.WithSampler(trace.AlwaysSample()))
+	}
+
+	traceProvider := trace.NewTracerProvider(o...)
 	otel.SetTracerProvider(traceProvider)
 
 	return SDK{