Expose an explicit Tracer.Flush() method

Extend the OpenTracing API with an explicit Flush() method to force
spans to be flushed eagerly without closing the tracer.

Signed-off-by: Craig Ringer <craig@2ndquadrant.com>

src/jaegertracing/Tracer.h

@@ -194,6 +194,11 @@ class Tracer : public opentracing::Tracer,
 
     void close() noexcept { Close(); }
 
+    void Flush()
+    {
+        _reporter->flush();
+    }
+
     const std::string& serviceName() const { return _serviceName; }
 
     const std::vector<Tag>& tags() const { return _tags; }

src/jaegertracing/reporters/RemoteReporter.h

@@ -48,13 +48,13 @@ class RemoteReporter : public Reporter {
 
     void close() override;
 
+    void flush() override;
+
   private:
     void sweepQueue();
 
     void sendSpan(const Span& span);
 
-    void flush();
-
     bool bufferFlushIntervalExpired() const
     {
         return (Clock::now() - _lastFlush) >= _bufferFlushInterval;

src/jaegertracing/reporters/Reporter.h

@@ -30,6 +30,8 @@ class Reporter {
     virtual void report(const Span& span) = 0;
 
     virtual void close() = 0;
+
+    virtual void flush() {};
 };
 
 }  // namespace reporters