[Request]: Add next observed data fallback

[toddtreece]

What are you really trying to do?

This was split as a separate request from #119 since it needs some additional discussion

There is a second customer request to have this behavior. They want this behavior + the same behavior for the next data point in the future.

For example: Chart shows no data in the time range visualized, but there is a data point in the past and a data point the future. The customer would like the line drawn between the two data points. This is important for step visualizations and other situations where state is displayed.

Context and constraints

Describe what you've already tried and where it falls short

Additional context

Environment

• Grafana version:
• Data source version:
• OS Grafana is installed on:
• User OS & Browser:
• Product or feature area:
• Impacted routes:
• Others:

[toddtreece]

is this something that could be solved with an interpolated query?

[toddtreece]

this one is a duplicate of #139