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

internal/config/config_test.go

@@ -737,6 +737,25 @@ func TestInstanceNameWithAControlCharacterStopsTheStartOnlyWithNtfySet(t *testin
 	}
 }
 
+func TestInstanceNameNotUTF8StopsTheStart(t *testing.T) {
+	t.Parallel()
+
+	// café saved in Latin-1.
+	const latin1 = "caf\xe9"
+
+	for name, env := range map[string]environment{
+		"set":       {instanceName: latin1},
+		"in a file": {instanceName + "_FILE": writeFile(t, latin1+"\n")},
+	} {
+		_, err := config.FromEnvironment(env.lookupEnv)
+
+		want := instanceName + `: "caf\xe9" is not valid UTF-8`
+		if err == nil || err.Error() != want {
+			t.Errorf("%s: error %v, want %s", name, err, want)
+		}
+	}
+}
+
 func TestCodeOnBothCountryListsStopsTheStart(t *testing.T) {
 	t.Parallel()