MINOR: [C++] Fix OTel 1.19 compat after silent breaking change (#45859)

### Rationale for this change

conda-forge/arrow-cpp-feedstock#1727 fails to build because open-telemetry/opentelemetry-cpp@762b73d is a silent breaking change.

### What changes are included in this PR?

Work around the breaking change.

### Are these changes tested?

I tested that it built with OTel 1.19 and 1.18.

### Are there any user-facing changes?

No

Lead-authored-by: David Li <li.davidm96@gmail.com>
Co-authored-by: Rossi Sun <zanmato1984@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>

Author: lidavidm

cpp/src/arrow/util/tracing_internal.cc

@@ -97,6 +97,11 @@ class OtlpOStreamExporter final : public sdktrace::SpanExporter {
                     std::chrono::microseconds(0)) noexcept override {
     return exporter_.Shutdown(timeout);
   }
+  // XXX: OTel 1.19 silent breaking change: this must be overridden
+  bool ForceFlush(std::chrono::microseconds /*timeout*/) noexcept override {
+    (*out_).flush();
+    return true;
+  }
 
  private:
   std::basic_ostream<char>* out_;