Cleanup usage of TimestampUnixNanos and its API

Updates #2488

Important Changes:

* Rename pdata.TimestampUnixNanos to pdata.Timestamp
* Remove pdata.TimestampUnixNanos and helpers, move them of the pdata.Timestamp type.
* Fix bug around IsZero, this function should return true if the time is January 1, year 1, 00:00:00 UTC not epoch.

Signed-off-by: Bogdan Drutu <bogdandrutu@gmail.com>

Author: bogdandrutu

consumer/pdata/common.go

@@ -19,19 +19,10 @@ package pdata
 
 import (
 	"sort"
-	"time"
 
 	otlpcommon "go.opentelemetry.io/collector/internal/data/protogen/common/v1"
 )
 
-// TimestampUnixNano is a time specified as UNIX Epoch time in nanoseconds since
-// 00:00:00 UTC on 1 January 1970.
-type TimestampUnixNano uint64
-
-func (ts TimestampUnixNano) String() string {
-	return time.Unix(0, int64(ts)).String()
-}
-
 // AttributeValueType specifies the type of AttributeValue.
 type AttributeValueType int