fix: make metadata.FromContext work again (#103)

FF Go used to have a dependency on the
"cloud.google.com/go/functions/metadata" package, which defined an
unexported context key and used it to add a value to the context. This
is the recommended way to add values to a context using
[WithValue](https://pkg.go.dev/context#WithValue).

FF Go has to continue using the `metadata.NewContext` call from
"cloud.google.com/go/functions/metadata" to have `metadata.FromContext`
from the same package work in customer's function code. This is due to
the fact that Golang purposely differentiates context keys from
different packages as different keys to prevent collisions, even if the
keys have the same string-name.

In the future, FF Go can move onto using its own `metadata` package to
avoid pulling in all the dependencies of
"cloud.google.com/go/functions/metadata", which contains other unrelated
GCF client code too. But it will have to be done with a breaking change
major version update.

funcframework/events.go

@@ -11,9 +11,9 @@ import (
 	"regexp"
 	"strings"
 
+	"cloud.google.com/go/functions/metadata"
 	"github.com/GoogleCloudPlatform/functions-framework-go/internal/events/pubsub"
 	"github.com/GoogleCloudPlatform/functions-framework-go/internal/fftypes"
-	"github.com/GoogleCloudPlatform/functions-framework-go/internal/metadata"
 )
 
 const (

funcframework/events_test.go

@@ -10,7 +10,7 @@ import (
 	"testing"
 	"time"
 
-	"github.com/GoogleCloudPlatform/functions-framework-go/internal/metadata"
+	"cloud.google.com/go/functions/metadata"
 	cloudevents "github.com/cloudevents/sdk-go/v2"
 	"github.com/google/go-cmp/cmp"
 	"github.com/google/go-cmp/cmp/cmpopts"

go.mod

@@ -3,6 +3,7 @@ module github.com/GoogleCloudPlatform/functions-framework-go
 go 1.11
 
 require (
+	cloud.google.com/go/functions v1.0.0
 	github.com/cloudevents/sdk-go/v2 v2.6.1
 	github.com/google/go-cmp v0.5.6
 )

internal/events/pubsub/pubsub.go

@@ -6,8 +6,8 @@ import (
 	"regexp"
 	"time"
 
+	"cloud.google.com/go/functions/metadata"
 	"github.com/GoogleCloudPlatform/functions-framework-go/internal/fftypes"
-	"github.com/GoogleCloudPlatform/functions-framework-go/internal/metadata"
 )
 
 const (

internal/events/pubsub/pubsub_test.go

@@ -5,8 +5,8 @@ import (
 	"testing"
 	"time"
 
+	"cloud.google.com/go/functions/metadata"
 	"github.com/GoogleCloudPlatform/functions-framework-go/internal/fftypes"
-	"github.com/GoogleCloudPlatform/functions-framework-go/internal/metadata"
 	"github.com/google/go-cmp/cmp"
 	"github.com/google/go-cmp/cmp/cmpopts"
 )

internal/fftypes/types.go

@@ -3,7 +3,7 @@
 package fftypes
 
 import (
-	"github.com/GoogleCloudPlatform/functions-framework-go/internal/metadata"
+	"cloud.google.com/go/functions/metadata"
 )
 
 // BackgroundEvent is the incoming payload to functions framework to trigger

testdata/conformance/function/function.go

@@ -23,8 +23,8 @@ import (
 	"log"
 	"net/http"
 
+	"cloud.google.com/go/functions/metadata"
 	"github.com/GoogleCloudPlatform/functions-framework-go/functions"
-	"github.com/GoogleCloudPlatform/functions-framework-go/internal/metadata"
 	cloudevents "github.com/cloudevents/sdk-go/v2"
 )