Add OpenTelemetry tracing support for Redis commands

- Add RedisSpan interface in rbcs-api for opaque span handles
- Extend TelemetryController with startRedisSpan/endRedisSpan methods
- Implement Redis tracing in rbcs-server-otel via OtelController and RedisOtelSpan
- Instrument RedisCacheHandler to create spans around GET and SET commands
- Add uses directive in rbcs-server-redis module-info for ServiceLoader discovery

Redis spans are created as CLIENT spans with attributes:
db.system=redis, db.operation=GET|SET, server.address, server.port

rbcs-server-otel/src/main/kotlin/net/woggioni/rbcs/server/otel/OtelController.kt

@@ -2,10 +2,13 @@ package net.woggioni.rbcs.server.otel
 
 import io.netty.channel.ChannelHandler
 import io.opentelemetry.api.GlobalOpenTelemetry
+import io.opentelemetry.api.trace.SpanKind
+import io.opentelemetry.api.trace.StatusCode
 import io.opentelemetry.instrumentation.logback.appender.v1_0.OpenTelemetryAppender
 import io.opentelemetry.instrumentation.netty.v4_1.NettyServerTelemetry
 import io.opentelemetry.instrumentation.runtimetelemetry.RuntimeTelemetry
 import io.opentelemetry.sdk.autoconfigure.AutoConfiguredOpenTelemetrySdk
+import net.woggioni.rbcs.api.RedisSpan
 import net.woggioni.rbcs.api.TelemetryController
 import net.woggioni.rbcs.common.createLogger
 import net.woggioni.rbcs.common.info
@@ -14,6 +17,10 @@ class OtelController : TelemetryController {
 
     private val log = createLogger<OtelController>()
 
+    private val tracer by lazy {
+        GlobalOpenTelemetry.getTracer("net.woggioni.rbcs.server.redis", "0.5.0")
+    }
+
     override fun initialize() {
         log.info { "Initializing OpenTelemetry SDK with auto-configuration" }
 
@@ -36,4 +43,24 @@ class OtelController : TelemetryController {
     override fun createHandler(): ChannelHandler {
         return NettyServerTelemetry.create(GlobalOpenTelemetry.get()).createCombinedHandler()
     }
+
+    override fun startRedisSpan(command: String, key: String): RedisSpan? {
+        val span = tracer.spanBuilder(command)
+            .setSpanKind(SpanKind.CLIENT)
+            .setAttribute("db.system", "redis")
+            .setAttribute("db.operation", command)
+            .startSpan()
+        return RedisOtelSpan(span)
+    }
+
+    override fun endRedisSpan(span: RedisSpan?) {
+        (span as? RedisOtelSpan)?.delegate?.end()
+    }
+
+    override fun endRedisSpan(span: RedisSpan?, error: Throwable) {
+        val s = (span as? RedisOtelSpan)?.delegate ?: return
+        s.recordException(error)
+        s.setStatus(StatusCode.ERROR)
+        s.end()
+    }
 }

rbcs-server-otel/src/main/kotlin/net/woggioni/rbcs/server/otel/RedisOtelSpan.kt

@@ -0,0 +1,17 @@
+package net.woggioni.rbcs.server.otel
+
+import io.opentelemetry.api.trace.Span
+import net.woggioni.rbcs.api.RedisSpan
+
+internal class RedisOtelSpan(
+    val delegate: Span,
+) : RedisSpan {
+
+    override fun setAttribute(key: String, value: String) {
+        delegate.setAttribute(key, value)
+    }
+
+    override fun setAttribute(key: String, value: Long) {
+        delegate.setAttribute(key, value)
+    }
+}