feat: [v2] Metrics cleanup

[kentquirk]

Which problem is this PR solving?

V2 metrics cleanup. V2 config rework gave us the ability to send metrics to multiple places. This adds that, plus a new OTel metrics target.

Short description of the changes

• HoneycombMetrics renamed to LegacyMetrics
• Multi-metrics object added so we can have metrics go to multiple destinations
• Fix a bug where stress relief wouldn't work if you don't have any metrics -- now it will
• Add OTel metrics destination
• Fetching config values can't error, so start removing some of the always-nil error returns to simplify things
• MultiMetrics tracks data for StressRelief (Store and Get)
• Injection system taught how to build a MultiMetrics object properly

Closes #562
Closes #581
Closes #446
Closes #351

It doesn't quite deal with #624 or #691 yet because the metrics being sent haven't been changed. Those will be a separate PR.