Feature/otel span kind support (#946)

* feat(otel): add span kind support to Lambda invocation tracing

* fix fmt

* docs(example): demonstrate OpenTelemetry span kind usage

* fix fmt

examples/opentelemetry-tracing/src/main.rs

@@ -1,5 +1,6 @@
 use lambda_runtime::{
     layers::{OpenTelemetryFaasTrigger, OpenTelemetryLayer as OtelLayer},
+    tracing::Span,
     LambdaEvent, Runtime,
 };
 use opentelemetry::trace::TracerProvider;
@@ -8,6 +9,8 @@ use tower::{service_fn, BoxError};
 use tracing_subscriber::prelude::*;
 
 async fn echo(event: LambdaEvent<serde_json::Value>) -> Result<serde_json::Value, &'static str> {
+    let span = Span::current();
+    span.record("otel.kind", "SERVER");
     Ok(event.payload)
 }
 

lambda-runtime/src/layers/otel.rs

@@ -4,7 +4,7 @@ use crate::LambdaInvocation;
 use opentelemetry_semantic_conventions::trace as traceconv;
 use pin_project::pin_project;
 use tower::{Layer, Service};
-use tracing::{instrument::Instrumented, Instrument};
+use tracing::{field, instrument::Instrumented, Instrument};
 
 /// Tower layer to add OpenTelemetry tracing to a Lambda function invocation. The layer accepts
 /// a function to flush OpenTelemetry after the end of the invocation.
@@ -75,6 +75,7 @@ where
         let span = tracing::info_span!(
             "Lambda function invocation",
             "otel.name" = req.context.env_config.function_name,
+            "otel.kind" = field::Empty,
             { traceconv::FAAS_TRIGGER } = &self.otel_attribute_trigger,
             { traceconv::FAAS_INVOCATION_ID } = req.context.request_id,
             { traceconv::FAAS_COLDSTART } = self.coldstart