Timestamp values are incorrectly rounded when retrieved from BigQuery #4031

Environment details

• OS: Windows
• .NET version: .NET Core 2.2
• Package name and version: Google.Cloud.BigQuery.V2 1.4.0

Steps to reproduce

1. Store value 2004-07-26 15:25:28.173333 in a timestamp column in BigQuery
2. Retrieve this value using BigQueryClient.ExecuteQueryAsync

The obtained value is 2004-07-26 15:25:28.173332

The issue is most likely in BigQueryRow.TimestampConverter. For the value above BigQuery returns 1090855528.1733329 as the number of seconds, which is 1090855528173332.9 microseconds. The decimal 9 is then truncated upon converting to long.