https://stackoverflow.com/a/58754984/401025

query.sql

#standardSQL
WITH `project.dataset.table` AS (
  SELECT 1 customer_id, -500 value, 
  DATE '2019-10-12' timestamp UNION ALL
  SELECT 1, -300, '2019-10-11' UNION ALL
  SELECT 1, -200, '2019-10-10' UNION ALL
  SELECT 2, 200, '2019-09-10' UNION ALL
  SELECT 2, 100, '2019-08-10' UNION ALL
  SELECT 2, 50, '2019-07-10' UNION ALL
  SELECT 1, 600, '2019-09-02' 
), customers AS (
  SELECT DISTINCT customer_id FROM `project.dataset.table`
), months AS (
  SELECT month FROM (
    SELECT DATE_TRUNC(MIN(timestamp), MONTH) min_month, DATE_TRUNC(MAX(timestamp), MONTH) max_month
    FROM `project.dataset.table`
  ), UNNEST(GENERATE_DATE_ARRAY(min_month, max_month, INTERVAL 1 MONTH)) month
)
SELECT customer_id, 
  IFNULL(value, LAG(value) OVER(win)) value,  
  IFNULL(timestamp, DATE_ADD(LAG(timestamp) OVER(win), INTERVAL DATE_DIFF(month, LAG(month) OVER(win), MONTH) MONTH)) timestamp  
FROM months, customers
LEFT JOIN (
  SELECT DATE_TRUNC(timestamp, MONTH) month, customer_id, 
    ARRAY_AGG(STRUCT(value, timestamp) ORDER BY timestamp DESC LIMIT 1)[OFFSET(0)].* 
  FROM `project.dataset.table` 
  GROUP BY month, customer_id
) USING(month, customer_id)
WINDOW win AS (PARTITION BY customer_id ORDER BY timestamp DESC)
-- ORDER BY month DESC, customer_id