Improve performance emitting LogRecords

- no need to clone current context for each processor
- use Context::map_current instead, outside loop, to
  compute optional TraceContext once

opentelemetry-api/src/trace/tracer.rs

@@ -169,7 +169,7 @@ pub trait Tracer {
 
     /// Start a [`Span`] from a [`SpanBuilder`].
     fn build(&self, builder: SpanBuilder) -> Self::Span {
-        self.build_with_context(builder, &Context::current())
+        Context::map_current(|cx| self.build_with_context(builder, cx))
     }
 
     /// Start a span from a [`SpanBuilder`] with a parent context.
@@ -382,7 +382,7 @@ impl SpanBuilder {
 
     /// Builds a span with the given tracer from this configuration.
     pub fn start<T: Tracer>(self, tracer: &T) -> T::Span {
-        tracer.build_with_context(self, &Context::current())
+        Context::map_current(|cx| tracer.build_with_context(self, cx))
     }
 
     /// Builds a span with the given tracer from this configuration and parent.

opentelemetry-sdk/src/logs/log_emitter.rs

@@ -5,7 +5,7 @@ use crate::{
 };
 use opentelemetry_api::{
     global::{self},
-    logs::{LogRecord, LogResult},
+    logs::{LogRecord, LogResult, TraceContext},
     trace::TraceContextExt,
     Context, InstrumentationLibrary,
 };
@@ -206,16 +206,19 @@ impl opentelemetry_api::logs::Logger for Logger {
             Some(provider) => provider,
             None => return,
         };
-
+        let trace_context = if self.include_trace_context {
+            Context::map_current(|cx| {
+                cx.has_active_span()
+                    .then(|| TraceContext::from(cx.span().span_context()))
+            })
+        } else {
+            None
+        };
         let config = provider.config();
         for processor in provider.log_processors() {
             let mut record = record.clone();
-            if self.include_trace_context {
-                let ctx = Context::current();
-                if ctx.has_active_span() {
-                    let span = ctx.span();
-                    record.trace_context = Some(span.span_context().into());
-                }
+            if let Some(ref trace_context) = trace_context {
+                record.trace_context = Some(trace_context.clone())
             }
             let data = LogData {
                 record,