add changes proposal and motivation

changes_proposal.md

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+1. cost tracker handles completion creation
+Change: separating completion and cost tracking, by changing the main functionality from `chat_completion` to `update_cost`
+Motivation:
+ - bulletproofs us from changes in how the completion is created, we only care about response structure
+ - allows easier integration, user only has to initialize tracker object and call `update_cost(response)`, 
+   otherwise each chat completion call would have to be rewritten 
+
+2. costs are calculated across all log files
+Change: 
+ - static `total_cost` that will calculate total spending from logs
+ - static `experiment_cost(experiment_name=self.experiment_name)` gets you total cost of specific experiment
+  - defaulting to current experiment_name in tracker object
+  - if object not initialized, experiment_name has to be provided
+ - `cost` that gets you costs for current run of this tracker object
+
+3. log file just acumulates total cost
+Change: 
+ - add breakdown of responses/input token per response/output token per response/cost per response
+ - maybe change log file format to json, so that we can better handle logs, for example:
+    ```
+    {
+        "experiment_name"
+        "model": 
+        "run_datetime":
+        "logs":
+        {
+            "0": {                                      # maybe datetime of response?
+                "num_of_input_tokens": 
+                "num_of_output_tokens":
+                "other":                                # additional info? message? thread? prompt?
+            }
+        }
+        "total": {
+            "cost":                                     # something else?
+        }
+    }
+    ```
+
+3. WIP