Prometheus exporter for IPFS API

[ghost]

We're about to rip prometheus out of go-ipfs, and need something to poll the API for this info.

Examples:

• ipfs swarm peers | wc -l
• :5001/debug/pprof

[jbenet]

too big?

[ghost]

While factoring out go-libp2p it became clear that the current integration isn't waterproof, i.e. go-libp2p would be depending on the prometheus client code. I didn't wanna justify this kind of bloat right at the birth of libp2p, and @whyrusleeping was happy enough to rip it out.

[jbenet]

Between no metrics and bloat we should be choosing bloat right now. gx should make it not a problem to depend on this. gx should only be downloading source code, so it should be pretty light. (and yes Go binaries are bloated in general)

[jbenet]

We should be adding more metrics, not removing them. I'm happy with whatever, but we shouldn't have a long interruption in metrics-- which is likely to happen if we don't make this an atomic swap (i.e. have something to replace it before removing it)

[ghost]

We have the proper IpfsCollector in go-ipfs now, which so far collects p2p_peers_total -- more possible metrics are:

• transport dimension for p2p_peers_total, e.g. transport=/ip6/udp/utp
• ipfs_gateway_request_inflight -- active gateway requests
• ipfs_api_request_inflight -- active API requests
• ipfs_repo_size -- disk space used by fs-repo
• ipfs_bitswap_wants -- number of wants
• ipfs_bitswap_bytes_rx -- incoming bytes counter
• ipfs_bitswap_bytes_tx -- outgoing bytes counter