There are cases when we can lost original timestamp field given from
logs, like when we send our logs to the next fluentd forwarder in chain
of forwarders, it will rewrite our timestamp by default. Save
`Timestamp` field explicitly to avoid such situation and be able to
reconstruct messages date and time.

Closes-Bug: #1781046
Change-Id: I2b4486aedacbe16dc4c0fb2e4e4984bd80e59f2d