opentelemetry-zipkin: Add Environment Variables

As described in https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/sdk-environment-variables.md
* Add support for:
** OTEL_EXPORTER_ZIPKIN_ENDPOINT
** OTEL_EXPORTER_ZIPKIN_TIMEOUT
* Update Default to use

Closes open-telemetry#534

opentelemetry-zipkin/Cargo.toml

@@ -38,6 +38,7 @@ http = "0.2"
 reqwest = { version = "0.11", optional = true, default-features = false }
 surf = { version = "2.0", optional = true, default-features = false }
 thiserror = { version = "1.0"}
+duration-str = "0.3.8"
 
 [dev-dependencies]
 bytes = "1"

opentelemetry-zipkin/src/exporter/env.rs

@@ -0,0 +1,58 @@
+use crate::ZipkinPipelineBuilder;
+
+/// HTTP endpoint for Zipkin collector.
+/// e.g. "http://localhost:9411/api/v2/spans"
+#[cfg(feature = "collector_client")]
+const ENV_ENDPOINT: &str = "OTEL_EXPORTER_ZIPKIN_ENDPOINT";
+
+/// Maximum time the Zipkin exporter will wait for each batch export
+#[cfg(feature = "collector_client")]
+const ENV_TIMEOUT: &str = "OTEL_EXPORTER_ZIPKIN_TIMEOUT";
+
+/// Assign builder attributes from env
+pub(crate) fn assign_attrs(builder: ZipkinPipelineBuilder) -> ZipkinPipelineBuilder {
+
+    #[cfg(any(
+        feature = "reqwest-client",
+        feature = "surf-client",
+        feature = "reqwest-blocking-client"
+    ))]
+    {
+        if let Some(endpoint) = env::var(ENV_ENDPOINT).ok().filter(|var| !var.is_empty()) {
+            builder = builder.with_collector_endpoint(endpoint);
+        }
+    }
+
+    #[cfg(any(
+        feature = "reqwest-client",
+        feature = "surf-client",
+        feature = "reqwest-blocking-client"
+    ))]
+    {
+        // TODO: Need to parse from strings
+        if let Some(timeout) = env::var(ENV_TIMEOUT).ok().filter(|var| !var.is_empty()) {
+            let timeout = duration_str::parse(timeout)
+                .expect(format!("{} is not formatted correctly.",ENV_TIMEOUT));
+
+            #[cfg(feature = "reqwest-blocking-client")]
+            let client = reqwest::blocking::Client::builder().timeout(timeout).build().unwrap();
+            #[cfg(all(
+                not(feature = "reqwest-blocking-client"),
+                not(feature = "surf-client"),
+                feature = "reqwest-client"
+            ))]
+            let client = reqwest::Client::builder().timeout(timeout).build().unwrap();
+            #[cfg(all(
+                not(feature = "reqwest-client"),
+                not(feature = "reqwest-blocking-client"),
+                feature = "surf-client"
+            ))]
+            let client: surf::Client = surf::Config::new()
+                .set_timeout(Some()).into();
+
+            builder = builder.with_http_client(client);
+        }
+    }
+
+    builder
+}

opentelemetry-zipkin/src/exporter/mod.rs

@@ -1,3 +1,4 @@
+mod env;
 mod model;
 mod uploader;
 
@@ -57,7 +58,7 @@ pub struct ZipkinPipelineBuilder {
 
 impl Default for ZipkinPipelineBuilder {
     fn default() -> Self {
-        ZipkinPipelineBuilder {
+        let builder_defaults = ZipkinPipelineBuilder {
             #[cfg(feature = "reqwest-blocking-client")]
             client: Some(Box::new(reqwest::blocking::Client::new())),
             #[cfg(all(
@@ -83,7 +84,10 @@ impl Default for ZipkinPipelineBuilder {
             service_addr: None,
             collector_endpoint: DEFAULT_COLLECTOR_ENDPOINT.to_string(),
             trace_config: None,
-        }
+        };
+
+        // Override above defaults with env vars if set
+        env::assign_attrs(builder_defaults)
     }
 }