feature: expose SimpleSpanProcessor::new

This commit update the visibility of the method `SimpleSpanProcessor::new` from `pub(crate)` to `pub`. This enables consumers to create `SimpleSpanProcessor` values for use with `trace::provider::Builder::with_span_processor`.

In particular, this fixes a consistency issue: the `BatchSpanProcessor` struct may already be built thanks to its `pub` builder. With this change, both processors in this repo may be built manually.

Closes open-telemetry#2060

opentelemetry-sdk/src/trace/span_processor.rs

@@ -109,7 +109,8 @@ pub struct SimpleSpanProcessor {
 }
 
 impl SimpleSpanProcessor {
-    pub(crate) fn new(exporter: Box<dyn SpanExporter>) -> Self {
+    /// Create a new [SimpleSpanProcessor] using the provided exporter.
+    pub fn new(exporter: Box<dyn SpanExporter>) -> Self {
         Self {
             exporter: Mutex::new(exporter),
         }