Improve rate limit instrumentation

Created by: ryanslade

We should include the following in our rate limit metrics for GitHub and GitLab:

• Consumed rate limit
• How much time we spent waiting for rate limiting in total
• How much time we spent waiting for rate limiting per-request
• How many requests we sent to the code host 👇
• How long requests to the code host took 👇

On investigation, I found that we already track code host request counts and durations here: https://github.com/sourcegraph/sourcegraph/blob/main/internal/metrics/metrics.go#L36

Used here for example: https://github.com/sourcegraph/sourcegraph/blob/main/internal/extsvc/github/common.go#L35

Part of: https://github.com/sourcegraph/sourcegraph/issues/8100