Add observability stack: Alloy, Tempo, and OTEL support

- Add NixOS modules for Alloy and Tempo with default configs
- Update Grafana datasource config for Prometheus, Loki, Tempo
- Add Prometheus remote_write for Alloy
- Implement OTEL metrics/tracing/logging in arrtrix (Go)
- Enable Alloy and Tempo in ulmo system config

packages/arrtrix/pkg/observability/config.go

@@ -0,0 +1,22 @@
+package observability
+
+import "strings"
+
+type Config struct {
+	OTLPGRPCEndpoint   string            `yaml:"otlp_grpc_endpoint"`
+	ServiceName        string            `yaml:"service_name"`
+	ResourceAttributes map[string]string `yaml:"resource_attributes"`
+}
+
+func (c *Config) ApplyDefaults() {
+	if c.ServiceName == "" {
+		c.ServiceName = "arrtrix"
+	}
+	if c.ResourceAttributes == nil {
+		c.ResourceAttributes = map[string]string{}
+	}
+}
+
+func (c Config) Enabled() bool {
+	return strings.TrimSpace(c.OTLPGRPCEndpoint) != ""
+}