plugins(stats_over_http): Add Prometheus v2 output with metric labels

The current Prometheus output in stats_over_http dumps flattened metric
names (e.g., proxy.process.http.200_responses), which makes it difficult
to aggregate data in Grafana or Prometheus. This adds a new "v2" output
format that parses these names into proper labels like
status="200" or method="GET".

The new format can be accessed via /_stats/prometheus_v2 or by passing
an Accept header with version=2.0.0.

To keep this efficient, the parsing uses swoc::TextView for a zero-
allocation approach. It recognizes common ATS patterns such as status
codes, HTTP methods, cache results, and thread/volume indices. I've
also added deduplication for the HELP and TYPE metadata lines, so that
metrics sharing the same base name (but different labels) are grouped
correctly in the output.

Includes compile-time unit tests (static_assert) to verify the parsing
logic and updated AuTests for the new endpoint.

Author: itsh3sam

doc/admin-guide/plugins/stats_over_http.en.rst

@@ -105,21 +105,29 @@ if you wish to have it in CSV format you can do so by passing an ``Accept`` head
 
 .. option:: Accept: text/csv
 
-Prometheus formatted output is also supported via the ``Accept`` header:
+Prometheus formatted output is also supported via the ``Accept`` header. Version 0.0.4
+(flat metric names) and version 2.0.0 (labeled metrics for better aggregation)
+are supported:
 
 .. option:: Accept: text/plain; version=0.0.4
+.. option:: Accept: text/plain; version=2.0.0
 
 Alternatively, the output format can be specified as a suffix to the configured
 path in the HTTP request target.  The supported suffixes are ``/json``,
-``/csv``, and ``/prometheus``.  For example, if the path is set to ``/_stats``
-(the default), you can access the stats in CSV format by using the URL::
+``/csv``, ``/prometheus``, and ``/prometheus_v2``.  For example, if the path
+is set to ``/_stats`` (the default), you can access the stats in CSV format by
+using the URL::
 
     http://host:port/_stats/csv
 
-The Prometheus format can be requested by using the URL::
+The Prometheus version 0.0.4 format (flat) can be requested by using the URL::
 
     http://host:port/_stats/prometheus
 
+The Prometheus v2 labeled format can be requested by using the URL::
+
+    http://host:port/_stats/prometheus_v2
+
 The JSON format is the default, but you can also access it explicitly by using the URL::
 
     http://host:port/_stats/json
@@ -129,9 +137,11 @@ specify a path suffix, the plugin will return the data in that format regardless
 the ``Accept`` header.
 
 In either case the ``Content-Type`` header returned by ``stats_over_http.so`` will
-reflect the content that has been returned: ``text/json``, ``text/csv``, or
-``text/plain; version=0.0.4; charset=utf-8`` for JSON, CSV, and Prometheus
-formats respectively.
+reflect the content that has been returned: ``text/json``, ``text/csv``,
+``text/plain; version=0.0.4; charset=utf-8``, or
+``text/plain; version=2.0.0; charset=utf-8`` for JSON, CSV, Prometheus v1, and
+Prometheus v2 formats respectively.
+
 
 Stats over http also accepts returning data in gzip or br compressed format per the
 ``Accept-encoding`` header. If the header is present, the plugin will return the