OpenTelemetry Protocol (OTLP) appender

Requires com.github.steffan-westcott/clj-otel-api.

With Java Agent:

Activate an appender configured by the OpenTelemetry Java Agent:
```clj
(let [logger-provider (.getLogsBridge (GlobalOpenTelemetry/get))
      appender (taoensso.timbre.appenders.community.otlp/otlp-appender logger-provider)]
  (taoensso.timbre/merge-config! {:appenders {:otlp appender}}))
```

Note: When relying on the OpenTelemetry Java Agent 1.x, you need
to explicitly enable the logs exporter with `OTEL_LOGS_EXPORTER=otlp`.
This will become the default with the release of Java Agent 2.0, cf.
* https://github.com/open-telemetry/opentelemetry-java-instrumentation/blob/main/CHANGELOG.md#version-1270-2023-06-14
* open-telemetry/opentelemetry-java-instrumentation#8647

Without Java Agent:

If you want autoconfiguration without the Java Agent, you also need
io.opentelemetry/opentelemetry-sdk-extension-autoconfigure and
io.opentelemetry/opentelemetry-exporter-otlp on the classpath.

Create an autoconfigured appender and activate it:
```clj
(let [logger-provider (.getSdkLoggerProvider
                        (.getOpenTelemetrySdk
                          (.build
                            (AutoConfiguredOpenTelemetrySdk/builder))))
      appender (taoensso.timbre.appenders.community.otlp/otlp-appender logger-provider)]
  (taoensso.timbre/merge-config! {:appenders {:otlp appender}}))
```

If you already have an instance of `GlobalOpenTelemetry`, e.g. created
by the OpenTelemetry Java Agent, you need to prevent setting the newly
created SDK as the global default:
```clj
(.build
  (doto (AutoConfiguredOpenTelemetrySdk/builder)
    (.setResultAsGlobal false)))
```

I took inspiration from `taoensso.timbre.appenders.community.sentry`.

Once steffan-westcott/clj-otel#8 is implemented,
the actual log emission should be replaced with using clj-otel's API.

project.clj

@@ -52,7 +52,7 @@
      [server-socket           "1.0.0"]
      [org.zeromq/cljzmq       "0.1.4"]
      [cljs-node-io            "1.1.2"] ; Node spit appender
-     ]}
+     [com.github.steffan-westcott/clj-otel-api "0.2.3"]]}
 
    :extra
    {:source-paths [         "src"                "extra/src"]

src/taoensso/timbre/appenders/community/otlp.clj

@@ -0,0 +1,105 @@
+(ns taoensso.timbre.appenders.community.otlp
+  "OpenTelemetry Protocol (OTLP) appender.
+  Requires com.github.steffan-westcott/clj-otel-api.
+
+  # With Java Agent
+
+  Activate an appender configured by the OpenTelemetry Java Agent:
+  ```clj
+  (let [logger-provider (.getLogsBridge (GlobalOpenTelemetry/get))
+        appender (otlp/appender logger-provider)]
+    (timbre/merge-config! {:appenders {:otlp appender}}))
+  ```
+
+  Note: When relying on the OpenTelemetry Java Agent 1.x, you need
+  to explicitly enable the logs exporter with `OTEL_LOGS_EXPORTER=otlp`.
+  This will become the default with the release of Java Agent 2.0, cf.
+  * https://github.com/open-telemetry/opentelemetry-java-instrumentation/blob/main/CHANGELOG.md#version-1270-2023-06-14
+  * https://github.com/open-telemetry/opentelemetry-java-instrumentation/pull/8647
+
+  # Without Java Agent
+
+  If you want autoconfiguration without the Java Agent, you also need
+  io.opentelemetry/opentelemetry-sdk-extension-autoconfigure and
+  io.opentelemetry/opentelemetry-exporter-otlp on the classpath.
+
+  Create an autoconfigured appender and activate it:
+  ```clj
+  (let [logger-provider (.getSdkLoggerProvider
+                          (.getOpenTelemetrySdk
+                            (.build
+                              (AutoConfiguredOpenTelemetrySdk/builder))))
+        appender (otlp/appender logger-provider)]
+    (timbre/merge-config! {:appenders {:otlp appender}}))
+  ```
+
+  If you already have an instance of `GlobalOpenTelemetry`, e.g. created
+  by the OpenTelemetry Java Agent, you need to prevent setting the newly
+  created SDK as the global default:
+  ```clj
+  (.build
+    (doto (AutoConfiguredOpenTelemetrySdk/builder)
+      (.setResultAsGlobal false)))
+  ```"
+  {:author "Dennis Schridde (@devurandom)"}
+  (:require
+    [steffan-westcott.clj-otel.api.attributes :as attr])
+  (:import
+    (io.opentelemetry.api.logs LoggerProvider Severity)
+    (java.util Date)))
+
+(set! *warn-on-reflection* true)
+
+(def ^:private default-severity
+  Severity/INFO)
+
+(def ^:private timbre->otlp-levels
+  {:trace  Severity/TRACE
+   :debug  Severity/DEBUG
+   :info   Severity/INFO
+   :warn   Severity/WARN
+   :error  Severity/ERROR
+   :fatal  Severity/FATAL
+   :report default-severity})
+
+(defn- single-map [xs]
+  (let [[x & r] xs]
+    (when (and (map? x) (not r))
+      x)))
+
+(defn appender
+  [^LoggerProvider logger-provider]
+  {:enabled?   true
+   :async?     true
+   :min-level  nil
+   :rate-limit nil
+   :output-fn  :inherit
+   :fn
+   (fn [{:keys [^Date instant level ^String ?ns-str ?file ?line ?err vargs msg_ context]}]
+     (let [actual-instant (.toInstant instant)
+           severity       (get timbre->otlp-levels level default-severity)
+           arg            (single-map vargs)
+           message        (if-let [msg (:msg arg)]
+                            msg
+                            (force msg_))
+           ?ex-data       (ex-data ?err)
+           extra          (cond-> context
+                                  (and ?file (not (contains? context :file)))
+                                  (assoc :file ?file)
+                                  (and ?line (not (contains? context :line)))
+                                  (assoc :line ?line)
+                                  (and ?ex-data (not (contains? context :ex-data)))
+                                  (assoc :ex-data ?ex-data))
+           event          (merge (dissoc arg :msg)
+                                 extra)
+           attributes     (attr/->attributes event)
+           ; TODO: Use clj-otel once it supports the logs API.
+           ;  cf. https://github.com/steffan-westcott/clj-otel/issues/8
+           logger         (.get logger-provider ?ns-str)]
+       (.emit
+         (doto (.logRecordBuilder logger)
+           (.setTimestamp actual-instant)
+           (.setSeverity severity)
+           (.setSeverityText (.toString severity))
+           (.setAllAttributes attributes)
+           (.setBody message)))))})