Cleanup usage of TimestampUnixNanos and its API (open-telemetry#2549)

Updates open-telemetry/opentelemetry-collector#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>

log.go

@@ -25,7 +25,7 @@ import (
 
 var (
 	TestLogTime      = time.Date(2020, 2, 11, 20, 26, 13, 789, time.UTC)
-	TestLogTimestamp = pdata.TimestampUnixNano(TestLogTime.UnixNano())
+	TestLogTimestamp = pdata.TimestampFromTime(TestLogTime)
 )
 
 func GenerateLogDataEmpty() pdata.Logs {

metric.go

@@ -23,13 +23,13 @@ import (
 
 var (
 	TestMetricStartTime      = time.Date(2020, 2, 11, 20, 26, 12, 321, time.UTC)
-	TestMetricStartTimestamp = pdata.TimestampUnixNano(TestMetricStartTime.UnixNano())
+	TestMetricStartTimestamp = pdata.TimestampFromTime(TestMetricStartTime)
 
 	TestMetricExemplarTime      = time.Date(2020, 2, 11, 20, 26, 13, 123, time.UTC)
-	TestMetricExemplarTimestamp = pdata.TimestampUnixNano(TestMetricExemplarTime.UnixNano())
+	TestMetricExemplarTimestamp = pdata.TimestampFromTime(TestMetricExemplarTime)
 
 	TestMetricTime      = time.Date(2020, 2, 11, 20, 26, 13, 789, time.UTC)
-	TestMetricTimestamp = pdata.TimestampUnixNano(TestMetricTime.UnixNano())
+	TestMetricTimestamp = pdata.TimestampFromTime(TestMetricTime)
 )
 
 const (

trace.go

@@ -24,13 +24,13 @@ import (
 
 var (
 	TestSpanStartTime      = time.Date(2020, 2, 11, 20, 26, 12, 321, time.UTC)
-	TestSpanStartTimestamp = pdata.TimestampUnixNano(TestSpanStartTime.UnixNano())
+	TestSpanStartTimestamp = pdata.TimestampFromTime(TestSpanStartTime)
 
 	TestSpanEventTime      = time.Date(2020, 2, 11, 20, 26, 13, 123, time.UTC)
-	TestSpanEventTimestamp = pdata.TimestampUnixNano(TestSpanEventTime.UnixNano())
+	TestSpanEventTimestamp = pdata.TimestampFromTime(TestSpanEventTime)
 
 	TestSpanEndTime      = time.Date(2020, 2, 11, 20, 26, 13, 789, time.UTC)
-	TestSpanEndTimestamp = pdata.TimestampUnixNano(TestSpanEndTime.UnixNano())
+	TestSpanEndTimestamp = pdata.TimestampFromTime(TestSpanEndTime)
 )
 
 func GenerateTraceDataEmpty() pdata.Traces {