fix metrics wiring.

[raulk]

Problem

Some components like go-ds-measure, go-ipfs-blockstore and go-bitswap expose metrics via ipfs/go-metrics-interface, but Lotus never injects the Prometheus exporter (ipfs/go-metrics-prometheus). Therefore, those metrics never surface in instrumentation.

Instead, Lotus uses OpenCensus directly.

Changes

This commit injects the Prometheus exporter for go-metrics-interface, and instructs the OpenCensus Prometheus exporter to use the DefaultRegistry.

This has the effect of exposing blending the metrics of both metrics libraries.

With this patch, the datastore, cache utilisation, and bitswap metrics are now exported via the /debug/metrics endpoint.

This commit also fixes an issue where the metrics scope was empty, making go-metrics-interface default to "". Angle brackets are inadmissible characters for Prometheus, so it was refusing to export the affected metrics. (These were the ARC cache metrics.)

[raulk]

Some new metrics

Old metrics are still there, of course

[lanzafame]

@raulk This looks good. Is there any way to have the fsrepo_* metrics be prefixed with lotus_? There is a chance for this to be exposed by IPFS on the same host, this problem can be solved on the querying side of things by creating graphs that sort by the job label which prometheus adds automatically when scraping but it would be the only one of all the lotus metrics so there will be confusion.

[raulk]

@lanzafame I investigated and unfortunately came up dry. Due to the way that measures are being registered, and the way go-metrics-interface is built, we'd need to change go-ds-metrics to take a context, which allows us to inject a custom namespace.

I suggest we merge this as is, and if it becomes a problem, we can look at doing that.

[lanzafame]

I figured the lack of context in that interface was the reason but I wanted to hope otherwise for a little bit