Draw Line In GameView Point to point

For this example, I'm using a Ray, RaycastHit, and Vector3.Distance. I'm simply creating a new LineRenderer object each time you click on 2 different colliders. This measures the distance between the hit point of each transform, not just the transforms themselves. If you just want the transforms' positions to be measured, remove h0 and h1 and just use p0.position and p1.position as the Vector3.Distance parameters. I just used a scene w/ some cubes for testing.

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class Example : MonoBehaviour 
 {
     Ray ray;
     RaycastHit hit;
     Transform p0 = null;
     Transform p1 = null;
     Vector3 h0;
     Vector3 h1;
     int i = 0;
     Transform container;
     float distance;
     string label = "";
     
     void Start()
     {
         container = new GameObject("Lines").transform;
     }
     
     void Update()
     {
         ray = Camera.main.ScreenPointToRay(Input.mousePosition);
         
         if(Physics.Raycast(ray, out hit) && Input.GetMouseButtonDown(0))
         {
             if(!p0 && !p1)
             {
                 p0 = hit.transform;
                 h0 = hit.point;
                 return;
             }
             
             if(p0 && !p1)
             {
                 p1 = hit.transform;
                 h1 = hit.point;
             }
                 
             if(p0 && p1)
             {
                 LineRenderer line = new GameObject("Line " + i.ToString()).AddComponent<LineRenderer>();
                 line.transform.parent = container;
                 line.SetWidth(0.025F, 0.025F);
                 line.SetColors(Color.red, Color.green);
                 line.SetVertexCount(2);
                 line.SetPosition(0, h0);
                 line.SetPosition (1, h1);
                 distance = Vector3.Distance(h0, h1);
                 label = "Distance between hit point 0 of " + p0.name + " and hit point 1 of " + p1.name + " = " + distance.ToString ();
                 p1 = null;
                 p0 = null;
             }
         }
     }
     
     void OnGUI()
     {
         if(GUILayout.Button ("Reset"))
         {
             foreach(Transform lineObject in container)
             {
                 Destroy(lineObject.gameObject);
             }
         }
         
         GUILayout.Label(label);
     }
 }