Method to add service_name to all spans

It's useful to have a way to set the service name for all spans at once,
rather than having to set it every time.

This commit adds a with_service_name method to PipelineBuilder
and Exporter to allow users to set the service name in one place,
following the behaviour of the official DataDog and Zipkin exporters.

It also sets a default service_name if the user does not configure one.

src/lib.rs

@@ -183,6 +183,9 @@ use std::convert::TryInto;
 use std::error::Error as StdError;
 use tags::{get_tags_for_event, get_tags_for_span};
 
+/// Default service name if no service is configured.
+const DEFAULT_SERVICE_NAME: &str = "unknown_service";
+
 /// Create a new Application Insights exporter pipeline builder
 pub fn new_pipeline(instrumentation_key: String) -> PipelineBuilder<()> {
     PipelineBuilder {
@@ -191,6 +194,7 @@ pub fn new_pipeline(instrumentation_key: String) -> PipelineBuilder<()> {
         endpoint: None,
         instrumentation_key,
         sample_rate: None,
+        service_name: DEFAULT_SERVICE_NAME.to_string(),
     }
 }
 
@@ -202,6 +206,7 @@ pub struct PipelineBuilder<C> {
     endpoint: Option<http::Uri>,
     instrumentation_key: String,
     sample_rate: Option<f64>,
+    service_name: String,
 }
 
 impl<C> PipelineBuilder<C> {
@@ -215,6 +220,7 @@ impl<C> PipelineBuilder<C> {
             endpoint: self.endpoint,
             instrumentation_key: self.instrumentation_key,
             sample_rate: self.sample_rate,
+            service_name: self.service_name,
         }
     }
 
@@ -288,6 +294,22 @@ impl<C> PipelineBuilder<C> {
             ..self
         }
     }
+
+    /// Assign the service name under which to group traces.
+    ///
+    /// This will be translated, along with the service namespace, to the Cloud Role Name.
+    ///
+    /// ```
+    /// # use opentelemetry::{KeyValue, sdk};
+    /// let tracer = opentelemetry_application_insights::new_pipeline("...".into())
+    ///     .with_client(reqwest::blocking::Client::new())
+    ///     .with_service_name("my-application")
+    ///     .install_simple();
+    /// ```
+    pub fn with_service_name<T: Into<String>>(mut self, name: T) -> Self {
+        self.service_name = name.into();
+        self
+    }
 }
 
 impl<C> PipelineBuilder<C>
@@ -368,6 +390,7 @@ pub struct Exporter<C> {
     endpoint: http::Uri,
     instrumentation_key: String,
     sample_rate: f64,
+    service_name: String,
 }
 
 impl<C> Exporter<C> {
@@ -380,6 +403,7 @@ impl<C> Exporter<C> {
                 .expect("hardcoded endpoint is valid uri"),
             instrumentation_key,
             sample_rate: 100.0,
+            service_name: DEFAULT_SERVICE_NAME.to_string(),
         }
     }
 
@@ -405,9 +429,28 @@ impl<C> Exporter<C> {
         self
     }
 
-    fn create_envelopes(&self, span: SpanData) -> Vec<Envelope> {
+    /// Assign the service name under which to group traces.
+    ///
+    /// This will be translated, along with the service namespace, to the Cloud Role Name.
+    pub fn with_service_name<T: Into<String>>(mut self, name: T) -> Self {
+        self.service_name = name.into();
+        self
+    }
+
+    fn create_envelopes(&self, mut span: SpanData) -> Vec<Envelope> {
         let mut result = Vec::with_capacity(1 + span.message_events.len());
 
+        if span
+            .attributes
+            .get(&semcov::resource::SERVICE_NAME)
+            .is_none()
+        {
+            span.attributes.insert(opentelemetry::KeyValue::new(
+                semcov::resource::SERVICE_NAME,
+                self.service_name.clone(),
+            ))
+        }
+
         let (data, tags, name) = match span.span_kind {
             SpanKind::Server | SpanKind::Consumer => {
                 let data: RequestData = (&span).into();