feat!: add ContextPair to StructuredLogger

Motivation
==========

`addContext` has one sharp edge on it that seems to come up a lot during
onboarding: if you want to add a single context element, the syntax is
unintuitive.

Say we want to add the context `"foo" -> "bar"` to a logger. Naturally,
one might reach for `addContext(pairs: (String, Shown)*)`, but alas,
after erasure we have

```scala
logger.addContext(
  "foo" -> "bar" // Tuple2
)
```

which doesn't conform to either of `Map` or `Seq`. This leaves us with
these options:

```scala
logger.addContext(
  Map(
    "foo" -> bar.show
  )
)

logger.addContext(
  "foo" -> (bar: Shown)
)
```

Neither feels _great_, and it would be really pleasant to have
consistent syntax for single-element and multi-element context data
without having to use `Map` literals and explicit `.show`/`.toString`.

Result
======

This patch hijacks `StructuredLogger`'s implicit scope to do a bit of
juggling and resolve the erasure issue, and in the process cleans up a
chunk of duplicative code in its descendants.

I've tested this in the work code, and it appears to work as expected
for any subtype of `StructuredLogger`. The signature ain't exactly
beautiful, but the syntactic consistency is gratifying.

Author: jhenahan

core/shared/src/main/scala/org/typelevel/log4cats/SelfAwareStructuredLogger.scala

@@ -17,7 +17,6 @@
 package org.typelevel.log4cats
 
 import cats.*
-import cats.Show.Shown
 
 trait SelfAwareStructuredLogger[F[_]] extends SelfAwareLogger[F] with StructuredLogger[F] {
   override def mapK[G[_]](fk: F ~> G): SelfAwareStructuredLogger[G] =
@@ -26,13 +25,6 @@ trait SelfAwareStructuredLogger[F[_]] extends SelfAwareLogger[F] with Structured
   override def addContext(ctx: Map[String, String]): SelfAwareStructuredLogger[F] =
     SelfAwareStructuredLogger.withContext(this)(ctx)
 
-  override def addContext(
-      pairs: (String, Shown)*
-  ): SelfAwareStructuredLogger[F] =
-    SelfAwareStructuredLogger.withContext(this)(
-      pairs.map { case (k, v) => (k, v.toString) }.toMap
-    )
-
   override def withModifiedString(f: String => String): SelfAwareStructuredLogger[F] =
     SelfAwareStructuredLogger.withModifiedString[F](this, f)
 }

core/shared/src/main/scala/org/typelevel/log4cats/StructuredLogger.scala

@@ -17,7 +17,9 @@
 package org.typelevel.log4cats
 
 import cats.*
+import cats.syntax.foldable.*
 import cats.Show.Shown
+import cats.Show.ContravariantShow
 
 trait StructuredLogger[F[_]] extends Logger[F] {
   def trace(ctx: Map[String, String])(msg: => String): F[Unit]
@@ -36,18 +38,27 @@ trait StructuredLogger[F[_]] extends Logger[F] {
   def addContext(ctx: Map[String, String]): StructuredLogger[F] =
     StructuredLogger.withContext(this)(ctx)
 
+  def addContext(pair: StructuredLogger.ContextPair): StructuredLogger[F] =
+    addContext(pair.toMap)
+
   def addContext(
-      pairs: (String, Shown)*
+      pairs: StructuredLogger.ContextPair*
   ): StructuredLogger[F] =
-    StructuredLogger.withContext(this)(
-      pairs.map { case (k, v) => (k, v.toString) }.toMap
-    )
+    addContext(pairs.foldMap(_.toMap))
 
   override def withModifiedString(f: String => String): StructuredLogger[F] =
     StructuredLogger.withModifiedString[F](this, f)
 }
 
 object StructuredLogger {
+
+  final case class ContextPair(value: (String, Shown)) extends AnyVal {
+    def toMap: Map[String, String] = Map(value._1 -> value._2.toString)
+  }
+
+  implicit def log4catsTupleToContextPair[A: ContravariantShow](pair: (String, A)): ContextPair =
+    ContextPair((pair._1, Shown.mat(pair._2)))
+
   def apply[F[_]](implicit ev: StructuredLogger[F]): StructuredLogger[F] = ev
 
   def withContext[F[_]](sl: StructuredLogger[F])(ctx: Map[String, String]): StructuredLogger[F] =

core/shared/src/main/scala/org/typelevel/log4cats/extras/DeferredSelfAwareStructuredLogger.scala

@@ -20,7 +20,7 @@ import cats.data.Chain
 import cats.effect.kernel.Resource.ExitCase
 import cats.effect.kernel.{Concurrent, Ref, Resource}
 import cats.syntax.all.*
-import cats.{~>, Show}
+import cats.~>
 import org.typelevel.log4cats.SelfAwareStructuredLogger
 
 /**
@@ -39,11 +39,6 @@ trait DeferredSelfAwareStructuredLogger[F[_]]
   override def addContext(ctx: Map[String, String]): DeferredSelfAwareStructuredLogger[F] =
     DeferredSelfAwareStructuredLogger.withContext(this)(ctx)
 
-  override def addContext(pairs: (String, Show.Shown)*): DeferredSelfAwareStructuredLogger[F] =
-    DeferredSelfAwareStructuredLogger.withContext(this)(
-      pairs.map { case (k, v) => (k, v.toString) }.toMap
-    )
-
   override def withModifiedString(f: String => String): DeferredSelfAwareStructuredLogger[F] =
     DeferredSelfAwareStructuredLogger.withModifiedString(this, f)
 }

core/shared/src/main/scala/org/typelevel/log4cats/extras/DeferredStructuredLogger.scala

@@ -16,7 +16,6 @@
 
 package org.typelevel.log4cats.extras
 
-import cats.Show.Shown
 import cats.data.Chain
 import cats.effect.kernel.Resource.ExitCase
 import cats.effect.kernel.{Concurrent, Ref, Resource}
@@ -56,11 +55,6 @@ trait DeferredStructuredLogger[F[_]] extends StructuredLogger[F] with DeferredLo
   override def addContext(ctx: Map[String, String]): DeferredStructuredLogger[F] =
     DeferredStructuredLogger.withContext(this, ctx)
 
-  override def addContext(
-      pairs: (String, Shown)*
-  ): DeferredStructuredLogger[F] =
-    DeferredStructuredLogger.withContext(this, pairs.map { case (k, v) => (k, v.toString) }.toMap)
-
   override def withModifiedString(f: String => String): DeferredStructuredLogger[F] =
     DeferredStructuredLogger.withModifiedString[F](this, f)
 }