Returns client-activity metrics for a given statistic.

    Args:
      statistic: The name of the statistic, which should also be a column in the
        'clients' table.
      day_buckets: A set of n-day-active buckets.
      cursor: MySQL cursor for executing queries.

def _CountClientStatisticByLabel(self, statistic, day_buckets, cursor):
    """Returns client-activity metrics for a given statistic.

    Args:
      statistic: The name of the statistic, which should also be a column in the
        'clients' table.
      day_buckets: A set of n-day-active buckets.
      cursor: MySQL cursor for executing queries.
    """
    day_buckets = sorted(day_buckets)
    sum_clauses = []
    ping_cast_clauses = []
    timestamp_buckets = []
    now = rdfvalue.RDFDatetime.Now()

    for day_bucket in day_buckets:
      column_name = "days_active_{}".format(day_bucket)
      sum_clauses.append(
          "CAST(SUM({0}) AS UNSIGNED) AS {0}".format(column_name))
      ping_cast_clauses.append(
          "CAST(c.last_ping > FROM_UNIXTIME(%s) AS UNSIGNED) AS {}".format(
              column_name))
      timestamp_bucket = now - rdfvalue.Duration.FromDays(day_bucket)
      timestamp_buckets.append(
          mysql_utils.RDFDatetimeToTimestamp(timestamp_bucket))

    query = """
    SELECT j.{statistic}, j.label, {sum_clauses}
    FROM (
      SELECT c.{statistic} AS {statistic}, l.label AS label, {ping_cast_clauses}
      FROM clients c
      LEFT JOIN client_labels l USING(client_id)
      WHERE c.last_ping IS NOT NULL AND l.owner_username = 'GRR'
    ) AS j
    GROUP BY j.{statistic}, j.label
    """.format(
        statistic=statistic,
        sum_clauses=", ".join(sum_clauses),
        ping_cast_clauses=", ".join(ping_cast_clauses))

    cursor.execute(query, timestamp_buckets)

    counts = {}
    for response_row in cursor.fetchall():
      statistic_value, client_label = response_row[:2]
      for i, num_actives in enumerate(response_row[2:]):
        if num_actives <= 0:
          continue
        stats_key = (statistic_value, client_label, day_buckets[i])
        counts[stats_key] = num_actives
    return counts