Options for exposing structured metrics in expvars

[aalpern]

This adds a few capabilities to the expvar support:

• Makes the exported percentiles configurable (I like having the 98th as well as 99th available)
• Adds an option to group all the metrics as a map under a common key (i.e. you can put them all in "metrics", as opposed to just at the root of the expvar JSON blob). A collector grabbing metrics from the /debug/metrics endpoint can then just process everything under the metrics key, rather than maintaining a list of non-metrics keys to exclude.
• Adds an option to output metrics as maps, grouping all the rates & percentiles, etc... into a single map per metric, instead of making them all into separate vars (Hi @mihasya! this might look familiar to you :dash: :ship:), which works better than flat for some metrics collectors.

The defaults are all set so there's no change from existing behavior.

Output Examples

Setting a root key:

    "metrics": {
        "http.client.postmark.errors": 0,
        "http.client.postmark.method.GET.50-percentile": 0,
        "http.client.postmark.method.GET.75-percentile": 0,
        "http.client.postmark.method.GET.95-percentile": 0,
        "http.client.postmark.method.GET.99-percentile": 0,
        "http.client.postmark.method.GET.999-percentile": 0,

Setting a root key and structured output:

    "metrics": {
        "http.client.postmark.errors": {
            "count": 0
        },
        "http.client.postmark.method.GET": {
            "50-percentile": 0,
            "75-percentile": 0,
            "95-percentile": 0,
            "99-percentile": 0,
            "999-percentile": 0,
            "count": 0,

[kamilchm]

I would love to see it on master. @rcrowley ?