[Guideline] what if I don't set startTime for cumulative metric datapoint

[hossain-rayhan]

We had a small discussion on todays Metrics SIG meeting. Adding here for better tracking.

It is recommended that for IntSum()/ Cumulative metrics, we should set the start time. So the open questions are-

[1] Do we want to make it a required field?
[2] What if I don't set the startTime in the receiver?
[3] What is a good value for this? Some receivers set the receiver_start_time. Does it sound good?

Tagging @andrewhsu @jmacd @bogdandrutu for visibility.

[andrewhsu]

from the issue triage mtg today, seems like this issue should be moved to opentelemetry-proto repo

[bogdandrutu]

This is something that we need a decision. @jrcamp I think we discussed this.

[jrcamp]

See #229

[jmacd]

Closing this in favor of #229.

[jmacd]

(Thank you @hossain-rayhan!)