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-api/src/main/java/net/woggioni/rbcs/api/RedisSpan.java

@@ -0,0 +1,10 @@
+package net.woggioni.rbcs.api;
+
+import org.jetbrains.annotations.NotNull;
+
+public interface RedisSpan {
+
+    void setAttribute(@NotNull String key, @NotNull String value);
+
+    void setAttribute(@NotNull String key, long value);
+}

rbcs-api/src/main/java/net/woggioni/rbcs/api/TelemetryController.java

@@ -2,8 +2,15 @@ package net.woggioni.rbcs.api;
 
 import io.netty.channel.ChannelHandler;
 import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
 public interface TelemetryController {
     void initialize();
     @NotNull ChannelHandler createHandler();
+
+    @Nullable RedisSpan startRedisSpan(@NotNull String command, @NotNull String key);
+
+    void endRedisSpan(@Nullable RedisSpan span);
+
+    void endRedisSpan(@Nullable RedisSpan span, @NotNull Throwable error);
 }