Instance name on process log lines and every metric (closes #91)

Process log lines carry instance, as request lines do; the instance name
is read before the other settings, so the line saying a setting is
invalid carries it too. Every metric, Go's and the process's included,
carries the label instance, set once on the registry. README.md says so,
and that Prometheus keeps it as exported_instance unless the scrape sets
honor_labels. An instance name that is not valid UTF-8 stops the start,
as the metrics library panics on such a label.

Tests that read metrics expect the label; one helper replaces the alert
tests' loops that wait for them.

Judgement call: the label is named instance, as in the log lines and
alerts, although Prometheus gives each target a label of that name.

Model: opus-5-5

README.md

@@ -210,9 +210,11 @@ effective settings are logged at start.
   `https`, a host and an optional port, and nothing more.
 - `SWWAF_INSTANCE_NAME` (default: the host's name, which docker sets to the
   first 12 characters of the container's id unless the deployment names one):
-  the name each request log line gives as `instance`. Set it, for example to
+  the name every log line and alert gives as `instance`, and every metric
+  carries as its label `instance` (see "Metrics" below). Set it, for example to
   `fsn1app1/gitea`, for a name that stays the same when a deploy replaces the
-  container, and that tells instances apart when several log to one place.
+  container, and that tells instances apart when several log to one place. A
+  name that is not valid UTF-8, such as one saved in Latin-1, stops the start.
 - `SWWAF_MODE` (default `enforce`): `enforce`, or `observe` to pass on the
   requests `smallwebwaf` would refuse and log what it would have done (see "What
   it does so far" above).
@@ -513,7 +515,7 @@ A field that does not apply to a request is left out of its line, apart from
 
 No body is logged, and no header but those above. `smallwebwaf`'s own messages
 (start, the settings, stop, errors) share the stream as JSON lines marked
-`"type":"process"`.
+`"type":"process"`, each with `instance` as a request's line has it.
 
 Go's HTTP server, on which `smallwebwaf` is built, reads a request's line and
 headers before `smallwebwaf` sees the request, and some requests end there,
@@ -897,6 +899,13 @@ empty directory or set `SWWAF_RULES_ENABLED=false`.
 format, for a scraper that sends `SWWAF_METRICS_TOKEN`, through traefik like any
 other request. No metric carries a client's address.
 
+Every metric below, Go's and the process's included, carries the label
+`instance`, `SWWAF_INSTANCE_NAME`, as a constant label set once on the registry
+the metrics are kept in, rather than as a label each metric declares. Prometheus
+gives each series it scrapes an `instance` label of its own, the address it
+scraped, and keeps this one as `exported_instance` unless the scrape sets
+`honor_labels: true`.
+
 - `smallwebwaf_requests_total`, `smallwebwaf_request_bytes_total` and
   `smallwebwaf_response_bytes_total`: requests, and their body bytes each way,
   by `status_class`, such as `2xx`, or `none` when nothing was sent, and by