rate_limit: YAML metrics node swaps the metric tag and prefix

[sinhaparth5]

RateLimiter::initializeMetrics() takes the tag before the prefix:

void initializeMetrics(uint type, std::string tag, std::string prefix = RATE_LIMITER_METRIC_PREFIX)

The remap path passes them in that order (txn_limiter.cc:182). The YAML path
does not (limiter.h:228):

std::string prefix = metrics["prefix"] ? metrics["prefix"].as<std::string>() : RATE_LIMITER_METRIC_PREFIX;
std::string tag    = metrics["tag"] ? metrics["tag"].as<std::string>() : name();

initializeMetrics(RATE_LIMITER_TYPE_SNI, prefix, tag);

metric_helper() builds the name as prefix + "." + type + "." + tag
(limiter.cc:60-74), so the two halves come out reversed. Every SNI limiter
configured with a metrics node gets the wrong metric name, including when
only one of the two keys is set, because the defaults get swapped along with
them.

Reproduce

selector:
  - sni: example.com
    limit: 100
    metrics:
      prefix: myprefix
      tag: mytag

traffic_ctl metric match rate reports:

mytag.sni.myprefix.queued

instead of myprefix.sni.mytag.queued.

With only tag: mytag set, the name is mytag.sni.plugin.rate_limiter.queued
rather than plugin.rate_limiter.sni.mytag.queued. With only
prefix: myprefix set it is example.com.sni.myprefix.queued rather than
myprefix.sni.example.com.queued.

Present on master (a2011c2), dating to #10559. No autest covers the metric
names, which is why it has gone unnoticed.