feat: make trace ID and span ID public on OtelData (#233)

## Motivation

Get the IDs from the `OtelData` struct.

Closes #227

## Solution

When the context is started, we can get the IDs.

There is however nothing to report when the context is not yet fully
built. But if it is not, there is not span ID to report and reporting
the parent span's trace ID from the builder may not be correct as that
may still change.

Author: mladedav

src/lib.rs

@@ -126,17 +126,52 @@ pub use layer::{layer, OpenTelemetryLayer};
 
 #[cfg(feature = "metrics")]
 pub use metrics::MetricsLayer;
+use opentelemetry::trace::TraceContextExt as _;
 pub use span_ext::OpenTelemetrySpanExt;
 
 /// Per-span OpenTelemetry data tracked by this crate.
 #[derive(Debug)]
-pub(crate) struct OtelData {
+pub struct OtelData {
     /// The state of the OtelData, which can either be a builder or a context.
     state: OtelDataState,
     /// The end time of the span if it has been exited.
     end_time: Option<SystemTime>,
 }
 
+impl OtelData {
+    /// Gets the trace ID of the span.
+    ///
+    /// Returns `None` if the context has not been built yet. This can be forced e.g. by calling
+    /// [`context`] on the span (not on `OtelData`) or if [context activation] was not explicitly
+    /// opted-out of, simply entering the span for the first time.
+    ///
+    /// [`context`]: OpenTelemetrySpanExt::context
+    /// [context activation]: OpenTelemetryLayer::with_context_activation
+    pub fn trace_id(&self) -> Option<opentelemetry::TraceId> {
+        if let OtelDataState::Context { current_cx } = &self.state {
+            Some(current_cx.span().span_context().trace_id())
+        } else {
+            None
+        }
+    }
+
+    /// Gets the span ID of the span.
+    ///
+    /// Returns `None` if the context has not been built yet. This can be forced e.g. by calling
+    /// [`context`] on the span (not on `OtelData`) or if [context activation] was not explicitly
+    /// opted-out of, simply entering the span for the first time.
+    ///
+    /// [`context`]: OpenTelemetrySpanExt::context
+    /// [context activation]: OpenTelemetryLayer::with_context_activation
+    pub fn span_id(&self) -> Option<opentelemetry::SpanId> {
+        if let OtelDataState::Context { current_cx } = &self.state {
+            Some(current_cx.span().span_context().span_id())
+        } else {
+            None
+        }
+    }
+}
+
 /// The state of the OpenTelemetry data for a span.
 #[derive(Debug)]
 #[allow(clippy::large_enum_variant)]