Add metrics

[guseggert]

This library should emit both OpenCensus metrics in both the client and server. We'll start with the standard set of metrics:

• Number of reqs
• Latency
• Errors

And modeled after what we've found useful for operating the Hydras: https://github.com/libp2p/hydra-booster/blob/master/providers/reframe.go#L49

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[guseggert]

For metric publishing and logging it'd be nice to include some context about which router had a problem (like its name, URL, or something like that). The APIs are designed in a way that make this pretty difficult. We can fix this in a subsequent update, will open an issue once I clarify what we need.

[guseggert]

plumbed into kubo: ipfs/kubo#9354