metrics: add prometheus back

With a proper IpfsCollector object and tests, this time.
The collector object makes it easy to add further metrics,
like e.g. bitswap wants/provs.

License: MIT
Signed-off-by: Lars Gierth <larsg@systemli.org>

package.json

@@ -30,6 +30,11 @@
       "hash": "QmZ4Qi3GaRbjcx28Sme5eMH7RQjGkt8wHxt2a65oLaeFEV",
       "name": "gogo-protobuf",
       "version": "0.0.0"
+    },
+    {
+      "hash": "QmdhsRK1EK2fvAz2i2SH5DEfkL6seDuyMYEsxKa9Braim3",
+      "name": "client_golang",
+      "version": "0.0.0"
     }
   ],
   "gxVersion": "0.4.0",
@@ -39,4 +44,4 @@
   "license": "",
   "name": "go-ipfs",
   "version": "0.4.0"
-}
+}