stdoutlog: Do not print timestamps when WithoutTimestamps is set

[XSAM]

Shouldn't these be

	Timestamp                 *time.Time
	ObservedTimestamp         *time.Time

To nicely handle WithoutTimestamps?

Here is why: https://go.dev/play/p/-a5jI4jOXSK

AFAIK this would be a different behaviour that we have for stdoutmetric and stdouttrace.

We can handle it via a separate issue and PR as we may have different opinions.

Originally posted by @pellared in #5172 (comment)

[pellared]

I have forgotten to mention that omitempty should be added e.g.

Timestamp         *time.Time `json:",omitempty"`
ObservedTimestamp *time.Time `json:",omitempty"`

[XSAM]

I am hesitant to implement this, as the data has been used in the stdoutmetric and stdouttrace is exported structure. Changing these structures would bring break changes.

opentelemetry-go/sdk/metric/metricdata/data.go

Lines 78 to 81 in 1ff2e71

	// StartTime is when the timeseries was started. (optional)
	StartTime time.Time `json:",omitempty"`
	// Time is the time when the timeseries was recorded. (optional)
	Time time.Time `json:",omitempty"`

opentelemetry-go/sdk/trace/tracetest/span.go

Lines 52 to 53 in 1ff2e71

	StartTime time.Time
	EndTime time.Time

If we only implement this only for stdoutlog, then the behavior is different from what we have for stdoutmetric and stdouttrace, which is strange.

[pellared]

Changing these structures would bring break changes.

From https://pkg.go.dev/go.opentelemetry.io/otel/exporters/stdout/stdoutmetric:

The exporter is intended to be used for testing and debugging, it is not meant for production use. Additionally, it does not provide an interchange format for OpenTelemetry that is supported with any stability or compatibility guarantees.

I would not consider it breaking.

But I also to not have a strong opinion to have this issue addressed.

It would be good to have other opinions and maybe discuss it during SIG meeting.