Rename the metrics new_api.md to api.md and updated links

[reyang]

This is the 2nd part of the change, the 1st part is covered by #1674.

Previously we've decided to make progress on the metrics API spec by creating a separate new_api.md doc just to make PR review easier.

Now that the metrics API spec is getting in shape, we've discussed in the Apr. 29th Metrics API/SDK spec SIG meeting that we will rename it to api.md in early May.

Changes

• Renamed new_api.md to api.md.
• Removed old_api.md.
• Made necessary updates to the links.

Related oteps

• https://github.com/open-telemetry/oteps/blob/main/text/metrics/0146-metrics-prototype-scenarios.md

[arminru]

@reyang I think if you do it in two commits (not sure if renaming is enough or if an explicit git mv is necessary), it should keep the file history intact. This way, one would also be able to find the respective PRs including context for each line using git blame.

[reyang]

@reyang I think if you do it in two commits (not sure if renaming is enough or if an explicit git mv is necessary), it should keep the file history intact. This way, one would also be able to find the respective PRs including context for each line using git blame.

@arminru thanks for calling this out. Please take a look at this which shows that git blame is smart enough to show all the history on the new API doc.

Update: after taking a closer look I noticed some lines actually confused git blame. So I'm splitting the PR into two parts as suggested by @arminru, and here goes the 1st part #1674.