Reduce scrape cardinality: class codes, per-(source,vip) histograms, byte histograms

Collapses the status-code dimension of the counter key into six class
lanes (1xx..5xx/unknown) so per-(source,vip) counter cardinality no
longer grows with the number of distinct three-digit responses nginx
serves. Histogram series drop the code label entirely and aggregate
across classes. Adds nginx_ipng_latency_total with a code class label
so average latency per class can still be computed off the scrape.
Adds nginx_ipng_bytes_{in,out} histograms with configurable boundaries
via the new ipng_stats_byte_buckets directive. Bumps JSON schema to 2.

Operators who need full three-digit-code resolution should consume the
ipng_stats_logtail stream off-host; the stats zone intentionally trades
that resolution for a bounded scrape size.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/01-module/01-e2e.robot

@@ -71,14 +71,14 @@ Direct traffic tagged
 
 # --- Status code tracking ---
 
-Per-code counters
-    [Documentation]    404 and 200 appear as distinct code= labels.
+Per-class code counters
+    [Documentation]    4xx and 2xx appear as class-bucketed code= labels.
     Docker Exec Ignore Rc    ${CLIENT1}    curl -s http://10.0.1.1:8080/notfound
     Docker Exec Ignore Rc    ${CLIENT1}    curl -s http://10.0.1.1:8080/notfound
     Wait For Flush
     ${output} =    Scrape With Filter    source_tag=cl1
-    Should Contain    ${output}    code="404"
-    Should Contain    ${output}    code="200"
+    Should Contain    ${output}    code="4xx"
+    Should Contain    ${output}    code="2xx"
 
 # --- Duration histogram ---