feat: track slot time and add performance stats accessors to session object

[milkshakeiii]

Exposed properties session.slot_millis_sum and session.bytes_processed_sum can now be used to estimate performance/work done in a session. See internal issue 340278185

[milkshakeiii]

---BIGQUERY USAGE REPORT---
bq_dataframes_covid_line_graphs.ipynb - query count: 666, bytes processed sum: 5762544, slot millis sum: 1850067
bq_dataframes_covid_line_graphs.ipynb - query count: 43, bytes processed sum: 385451, slot millis sum: 1027972
bq_dataframes_covid_line_graphs.ipynb - query count: 40, bytes processed sum: 417171, slot millis sum: 497821
bq_dataframes_covid_line_graphs.ipynb - query count: 59, bytes processed sum: 563470, slot millis sum: 271811
bq_dataframes_covid_line_graphs.ipynb - query count: 12, bytes processed sum: 26556, slot millis sum: 100499
bq_dataframes_covid_line_graphs.ipynb - query count: 43, bytes processed sum: 10627962657, slot millis sum: 6160278
bq_dataframes_covid_line_graphs.ipynb - query count: 100, bytes processed sum: 14048626, slot millis sum: 653889
bq_dataframes_covid_line_graphs.ipynb - query count: 44, bytes processed sum: 5598362, slot millis sum: 176218
bq_dataframes_covid_line_graphs.ipynb - query count: 21, bytes processed sum: 810, slot millis sum: 144144
bq_dataframes_covid_line_graphs.ipynb - query count: 6, bytes processed sum: 34430816436, slot millis sum: 8633061
bq_dataframes_covid_line_graphs.ipynb - query count: 5, bytes processed sum: 26270652702, slot millis sum: 42959570
---total queries: 1039, total bytes: 71356234785, total slot millis: 62475330---

[chelsea-lin]

LGTM with a comment for potential further extensions.