Previously when an exception occurred in Sidekiq, Sidekiq would export
logs with timestamps (e.g. created_at, enqueued_at) in floating point
seconds, while other jobs would report in ISO 8601 format. This
inconsistency in data types would cause Elasticsearch to drop logs that
did not match the schema type (date in most cases).

This commit moves the responsibility of formatting timestamps to the
Sidekiq JSON formatter where it properly belongs. The job logger now
generates timestamps with floats, just as Sidekiq does. This ensures
that timestamps are manipulated only in one place.

See https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/8269