fix(telemetry): correct service.name label and make alerting boot without env

- MetricsRegistry: service.name was recorded as boolean true because the
  string literal "xrpld" bound to the OTel AttributeValue variant's
  char-const* -> bool overload. Assign std::string so it selects the
  string alternative; Prometheus now shows service_name=xrpld (and
  exported_job round-trips correctly) on every MetricsRegistry series.
- Grafana alerting contact points: the Slack url / email addresses used
  ${ENV_VAR} references that expand to empty when unset, and Grafana's
  provisioning validator (which lacks ${VAR:-default} support) then
  crashes the whole stack on startup. Use non-empty disabled placeholders
  (unroutable webhook host, .invalid email) so the stack boots with zero
  configuration; delivery stays off until a real destination is supplied.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/xrpld/telemetry/MetricsRegistry.cpp

@@ -182,7 +182,11 @@ MetricsRegistry::start(std::string const& endpoint, std::string const& instanceI
     // Configure resource attributes so Prometheus exported_instance labels
     // distinguish metrics from different nodes (matches OTelCollector setup).
     resource::ResourceAttributes attrs;
-    attrs[opentelemetry::semconv::service::kServiceName] = "xrpld";
+    // Use std::string, not a string literal: ResourceAttributes stores an
+    // OTel AttributeValue variant whose char-const* overload binds to bool,
+    // so "xrpld" would be recorded as the boolean true. std::string selects
+    // the string alternative and the value round-trips as service.name=xrpld.
+    attrs[opentelemetry::semconv::service::kServiceName] = std::string("xrpld");
     if (!instanceId.empty())
         attrs[opentelemetry::semconv::service::kServiceInstanceId] = instanceId;
     auto resourceAttrs = resource::Resource::Create(attrs);