Feature/3247 json metric type - add field "metric_type" to metric.json file

src/stan/mcmc/hmc/base_hmc.hpp

@@ -71,6 +71,7 @@ class base_hmc : public base_mcmc {
     struct_writer.begin_record();
     struct_writer.write("stepsize", get_nominal_stepsize());
     struct_writer.write("inv_metric", z_.inv_e_metric_);
+    struct_writer.write("metric_type", z_.metric_type());
     struct_writer.end_record();
   }
 

src/stan/mcmc/hmc/hamiltonians/dense_e_point.hpp

@@ -51,6 +51,8 @@ class dense_e_point : public ps_point {
       writer(inv_e_metric_ss.str());
     }
   }
+
+  inline std::string metric_type() { return "dense_e"; }
 };
 
 }  // namespace mcmc

src/stan/mcmc/hmc/hamiltonians/diag_e_point.hpp

@@ -49,6 +49,8 @@ class diag_e_point : public ps_point {
       inv_e_metric_ss << ", " << inv_e_metric_(i);
     writer(inv_e_metric_ss.str());
   }
+
+  inline std::string metric_type() { return "diag_e"; }
 };
 
 }  // namespace mcmc

src/stan/mcmc/hmc/hamiltonians/softabs_point.hpp

@@ -43,6 +43,8 @@ class softabs_point : public ps_point {
   virtual inline void write_metric(stan::callbacks::writer& writer) {
     writer("No free parameters for SoftAbs metric");
   }
+
+  inline std::string metric_type() { return "softabs"; }
 };
 
 }  // namespace mcmc

src/stan/mcmc/hmc/hamiltonians/unit_e_point.hpp

@@ -30,6 +30,8 @@ class unit_e_point : public ps_point {
   inline void write_metric(stan::callbacks::writer& writer) {
     writer("No free parameters for unit metric");
   }
+
+  inline std::string metric_type() { return "unit_e"; }
 };
 
 }  // namespace mcmc