add miss file text_map_carrier.h for tars tracking

servant/servant/text_map_carrier.h

@@ -0,0 +1,66 @@
+#ifndef LIGHTSTEP_TEXT_MAP_CARRIER
+#define LIGHTSTEP_TEXT_MAP_CARRIER
+
+#include <opentracing/propagation.h>
+#include <string>
+#include <unordered_map>
+
+using opentracing::expected;
+using opentracing::string_view;
+using opentracing::TextMapReader;
+using opentracing::TextMapWriter;
+
+class TextMapCarrier : public TextMapReader, public TextMapWriter {
+public:
+  TextMapCarrier(std::unordered_map<std::string, std::string> &text_map)
+      : text_map_(text_map) {}
+
+  expected<void> Set(string_view key, string_view value) const override {
+    text_map_[key] = value;
+    return {};
+  }
+
+  expected<void> ForeachKey(
+      std::function<expected<void>(string_view key, string_view value)> f)
+      const override {
+    for (const auto &key_value : text_map_) {
+      auto result = f(key_value.first, key_value.second);
+      if (!result)
+        return result;
+    }
+    return {};
+  }
+
+private:
+  std::unordered_map<std::string, std::string> &text_map_;
+};
+
+
+// span context reader/writer helper
+
+static std::string read_span_context(std::unordered_map<std::string, std::string> m)
+{
+    std::string output = "";
+
+    for (auto it = m.cbegin(); it != m.cend(); it++)
+    {
+        output += (it->first) + ":" + it->second + ",";
+    }
+
+    return output.substr(0, output.size() - 1);
+}
+
+
+static void write_span_context(std::unordered_map<std::string, std::string> &m, char *context)
+{
+   char * keypair = strtok(context, ",");
+   while(keypair != NULL)
+   {
+       std::string s(keypair);
+       std::string::size_type found = s.find_first_of(':');
+       m.insert(std::pair<std::string, std::string>(s.substr(0,found),s.substr(found+1)));
+       keypair =strtok(NULL, ",");
+   }
+}
+
+#endif // LIGHTSTEP_TEXT_MAP_CARRIER