message: add PrometheusTarget
diff: @@ -84,6 +84,33 @@ public:
  }
  };
  +class PrometheusTarget : public Target
  +{
  + std::string legend, expr;
  + PrometheusTarget() = delete;
  +public:
  + PrometheusTarget(const std::string& legend_, const std::string& expr_) : legend(legend_), expr(expr_) {}
  + std::string operator () (const std::string& refId) const
  + {
  + std::string result;
  + result += R"PCMDELIMITER(
  + {
  + "expr": ")PCMDELIMITER";
  + result += expr;
  + result += R"PCMDELIMITER(",
  + "instant": false,
  + "interval": "",
  + "legendFormat": ")PCMDELIMITER";
  + result += legend;
  + result += R"PCMDELIMITER(",
  + "refId": ")PCMDELIMITER";
  + result += refId;
  + result += R"PCMDELIMITER("
  + })PCMDELIMITER";
  + return result;
  + }
  +};
  +
  class Panel
  {
  int x, y, w, h;
  