[RFC] report: encode timestamps in a compact form

[alban]

This patch changes the encoding of two kinds of timestamps:

• timestamps in the "latest" map
• timestamps in metrics

We use Golang's internal time.Time marshalling and then base64 on it.

Before this patch, this is how it looked:

"host_node_id" : {
   "value" : "neptune;<host>",
   "timestamp" : "2017-04-07T15:50:55.320041355Z"
},
...
"samples" : [
   {
      "date" : "2017-04-07T17:50:40.284167472+02:00",
      "value" : 249266176
   },

After this patch, this is how it looks:

"latest" : {
   "host_node_id" : {
      "t" : "AQAAAA7QgZARDlWbvv//",
      "value" : "neptune;<host>"
   },
...
"samples" : [
   {
      "v" : 24.330900243309,
      "t" : "AQAAAA7QgZANEJKTv///"
   },
   {
      "v" : 25.9842519685039,
      "t" : "AQAAAA7QgZAODml7Af//"
   },

On my laptop with one Docker container running and 392 processes
running, I see 17815 timestamps in the report.

This should make the report smaller in its uncompressed form.

However, I should check if it makes the reports bigger in its compressed
form. It might happen because base64 strings are more difficult to
compress. I have not tested this yet. See:
http://stackoverflow.com/questions/38124361/why-does-base64-encoded-data-compress-so-bad

---

Addresses bug #1201

TODO:

• compare the report size (both compressed and uncompressed)
• check backward compatibility
• check cpu impact

/cc @ekimekim @bboreham

• Do you think it worth the complexity?

[bboreham]

Where are you seeing json reports? I thought all the ones used for real are msgpack now.

[bboreham]

and no, I don't think this is worth it, sorry

[ekimekim]

Agreed. Unless there's a visible perf improvement, this isn't worth the extra complexity. For one thing, it irrevocably ties the report format to golang, which matters for people writing plugins.

[alban]

Where are you seeing json reports? I thought all the ones used for real are msgpack now.

Yes, it uses msgpack. I just got the snippets in json form by clicking on the "download json" debug button in the Scope UI.

Fair enough, I'm closing this PR.