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.
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 "<no-scope>". Angle brackets are inadmissible
characters for Prometheus, so it was refusing to export the affected metrics.
(These were the ARC cache metrics.)
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