Store tags in StartSpanOptions as string. Correct comment. (#19)

* Small correct to the documentation of Start Span.

* Store span options as strings to prevent dangling pointer and to allow moving over the tags vector.

include/opentracing/tracer.h

@@ -25,7 +25,7 @@ struct StartSpanOptions {
   SystemTime start_system_timestamp;
   SteadyTime start_steady_timestamp;
   std::vector<std::pair<SpanReferenceType, const SpanContext*>> references;
-  std::vector<std::pair<string_view, Value>> tags;
+  std::vector<std::pair<std::string, Value>> tags;
 };
 
 // StartSpanOption instances (zero or more) may be passed to Tracer.StartSpan.
@@ -63,12 +63,12 @@ class Tracer {
   //     // The vanilla child span case:
   //     auto span = tracer.StartSpan(
   //         "GetFeed",
-  //         {opentracing::ChildOf(parentSpan.context())})
+  //         {opentracing::ChildOf(&parentSpan.context())})
   //
   //     // All the bells and whistles:
   //     auto span = tracer.StartSpan(
   //         "GetFeed",
-  //         {opentracing::ChildOf(parentSpan.context()),
+  //         {opentracing::ChildOf(&parentSpan.context()),
   //         opentracing::Tag{"user_agent", loggedReq.UserAgent},
   //         opentracing::StartTimestamp(loggedReq.timestamp())})
   //
@@ -245,7 +245,7 @@ class SetTag : public StartSpanOption {
   }
 
  private:
-  string_view key_;
+  const string_view key_;
   const Value& value_;
 };
 END_OPENTRACING_ABI_NAMESPACE