Remove effect constraint from TextMapPropagator (#332)

* Remove effect constraint from TextMapPropagator, update packages, add tests

* Make `ContextPropagators` sealed

* Rename `create` -> `of`, add `Monoid[TextMapPropagator[Ctx]]`

* TextMapPropagator - `fields: List[String]` -> `fields: Iterable[String]`

* Run `scalafixAll`

* Update java/common/src/main/scala/org/typelevel/otel4s/java/TextMapPropagatorImpl.scala

Co-authored-by: Marissa | April <[email protected]>

* TextMapPropagatorImpl: access implicit `TextMapUpdater` directly

* TextMapPropagator: keep unique fields when making a multi instance

---------

Co-authored-by: Marissa | April <[email protected]>

core/all/src/main/scala/org/typelevel/otel4s/Otel4s.scala

@@ -16,13 +16,14 @@
 
 package org.typelevel.otel4s
 
+import org.typelevel.otel4s.context.propagation.ContextPropagators
 import org.typelevel.otel4s.metrics.MeterProvider
 import org.typelevel.otel4s.trace.TracerProvider
 
 trait Otel4s[F[_]] {
   type Ctx
 
-  def propagators: ContextPropagators[F, Ctx]
+  def propagators: ContextPropagators[Ctx]
 
   /** A registry for creating named meters.
     */

core/common/src/main/scala/org/typelevel/otel4s/context/propagation/ContextPropagators.scala

@@ -0,0 +1,87 @@
+/*
+ * Copyright 2022 Typelevel
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.typelevel.otel4s.context.propagation
+
+/** A container of the registered propagators for every supported format.
+  *
+  * @tparam Ctx
+  *   the type of the Context
+  */
+sealed trait ContextPropagators[Ctx] {
+
+  /** Returns a text map propagator to extract or inject data.
+    */
+  def textMapPropagator: TextMapPropagator[Ctx]
+}
+
+object ContextPropagators {
+
+  /** Creates a [[ContextPropagators]] which can be used to extract and inject
+    * context in text payloads with the given [[TextMapPropagator]].
+    *
+    * If multiple text map propagators are passed, the combined (composite)
+    * TextMapPropagator instance will be created.
+    *
+    * It's a shortcut for:
+    * {{{
+    * ContextPropagators.of(TextMapPropagator.of(w3cPropagator, httpTracePropagator))
+    * }}}
+    *
+    * @example
+    *   {{{
+    * val w3cPropagator: TextMapPropagator[Context] = ???
+    * val httpTracePropagator: TextMapPropagator[Context] = ???
+    * val contextPropagators = ContextPropagators.of(w3cPropagator, httpTracePropagator)
+    *   }}}
+    *
+    * @see
+    *   [[TextMapPropagator.of]]
+    *
+    * @param textMapPropagators
+    *   the propagators to use for injection and extraction
+    *
+    * @tparam Ctx
+    *   the context to use to extract or inject data
+    */
+  def of[Ctx](
+      textMapPropagators: TextMapPropagator[Ctx]*
+  ): ContextPropagators[Ctx] =
+    new Default(TextMapPropagator.of(textMapPropagators: _*))
+
+  /** Creates a no-op implementation of the [[ContextPropagators]].
+    *
+    * A [[TextMapPropagator]] has no-op implementation too.
+    */
+  def noop[Ctx]: ContextPropagators[Ctx] =
+    new Noop
+
+  private final class Noop[Ctx] extends ContextPropagators[Ctx] {
+    val textMapPropagator: TextMapPropagator[Ctx] =
+      TextMapPropagator.noop
+
+    override def toString: String =
+      "ContextPropagators.Noop"
+  }
+
+  private final class Default[Ctx](
+      val textMapPropagator: TextMapPropagator[Ctx]
+  ) extends ContextPropagators[Ctx] {
+    override def toString: String =
+      s"ContextPropagators.Default{textMapPropagator=${textMapPropagator.toString}}"
+  }
+
+}

core/common/src/main/scala/org/typelevel/otel4s/TextMapGetter.scala → core/common/src/main/scala/org/typelevel/otel4s/context/propagation/TextMapGetter.scala

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-package org.typelevel.otel4s
+package org.typelevel.otel4s.context.propagation
 
 import cats.Contravariant