Allow Unix epoch (1970) as default starttimestamp

[danelowe]

It appears as though the opentelemetry-collector might be adding 1970-01-01 as the default value for StartTimestamp on TimeSeries DataPoints.

opentelemetry-operations-go/exporter/collector/metrics.go

Line 740 in b068af9

if (!ok && point.StartTimestamp().AsTime().IsZero()) || !point.StartTimestamp().AsTime().Before(point.Timestamp().AsTime()) {

Here the code expects it to be 0001-01-01, resulting in data not being sent to Google Cloud.

Also opentelemetry-collector stores timestamps as unsigned integers from 1970, so can't set the StartTimestamp to 0001-01-01 without counting on integer overflows.

open-telemetry/opentelemetry-collector#5102

[dashpole]

This was fixed in #336