title: Pandas DataFrame -add rows for missing months
body: <p>Hi I have a dataframe like below</p>
  <pre><code>  Cust_ID   created_date   tran_date        Sales_Value  Quantity_Sold
  0        1   2021-01-31    2021-01-31         126             12
  1        1   2021-01-31    2021-03-31           5              3
  2        1   2021-01-31    2021-06-30          20              5
  3        2   2020-12-31    2021-02-28         200              5
  4        2   2020-12-31    2021-03-31         250             25
  </code></pre>
  <p>I need to fill the missing months from the created date to till current month for each customer_ID. basically for each customer I should have one row for each month/year since the customer created date to the current date
  so the dataframe should look like below if current month is 07/2021</p>
  <pre><code>    Cust_ID   created_date   tran_date        Sales_Value  Quantity_Sold
  0        1   2021-01-31    2021-01-31         126             12
  1        1   2021-01-31    2021-02-28           0              0
  2        1   2021-01-31    2021-03-31           5              3
  3        1   2021-01-31    2021-04-30           0              0
  4        1   2021-01-31    2021-05-31           0              0
  5        1   2021-01-31    2021-06-30          20              5
  6        1   2021-01-31    2021-07-31           0              0
  7        2   2020-12-31    2020-12-31           0              0
  8        2   2020-12-31    2021-01-31           0              0
  9        2   2020-12-31    2021-02-28         200              5
  10       2   2020-12-31    2021-03-31         250             25
  11       2   2020-12-31    2021-04-30           0              0
  12       2   2020-12-31    2021-05-31           0              0
  13       2   2020-12-31    2021-06-30           0              0
  14       2   2020-12-31    2021-07-31           0              0
  </code></pre>
  <p>what's the best way to achieve this?</p>