Change metadata lookup to be case insensitive

Before this change, users would make references to headers such as "authorization" and "X-CloudFront-Viewer-Latitude", which would potentially fail, as they would be stored in the context as "X-Cloudfront-Viewer-Latitude" or "Authorization".

This PR changes this behavior, to attempt a case insensitive lookup to the backing map in case the key wasn't found under the requested casing. Under the assumption that the lookup key specified by users won't change, on a subsequent lookup, we proactively copy the value to the looked up key, to make the next lookup faster.

Fixes #5610
Fixes open-telemetry/opentelemetry-collector-contrib#8994

Signed-off-by: Juraci Paixão Kröhling <[email protected]>

client/client.go

@@ -91,6 +91,7 @@ package client // import "go.opentelemetry.io/collector/client"
 import (
 	"context"
 	"net"
+	"strings"
 )
 
 type ctxKey struct{}
@@ -160,7 +161,19 @@ func NewMetadata(md map[string][]string) Metadata {
 func (m Metadata) Get(key string) []string {
 	vals := m.data[key]
 	if len(vals) == 0 {
-		return nil
+		// we didn't find the key, but perhaps it just has different cases?
+		for k, v := range m.data {
+			if strings.EqualFold(key, k) {
+				vals = v
+				// we optimize for the next lookup
+				m.data[key] = v
+			}
+		}
+
+		// if it's still not found, it's really not here
+		if len(vals) == 0 {
+			return nil
+		}
 	}
 
 	ret := make([]string, len(vals))

client/client_test.go

@@ -91,6 +91,7 @@ func TestMetadata(t *testing.T) {
 	source := map[string][]string{"test-key": {"test-val"}}
 	md := NewMetadata(source)
 	assert.Equal(t, []string{"test-val"}, md.Get("test-key"))
+	assert.Equal(t, []string{"test-val"}, md.Get("test-KEY")) // case insensitive lookup
 
 	// test if copy. In regular use, source cannot change
 	val := md.Get("test-key")