Allow padding to be omitted from binary metadata values (#2243)

Motivation:

Binary metadata values are encoded as base64 strings. The gRPC spec
doesn't require that the values are padded. Currently gRPC Swift
requires values to be padded otherwise decoding will fail.

Modifications:

- Allow padding characters to be omitted when decoding base64

Result:

Can decode unpadded binary metadata values

IntegrationTests/Benchmarks/Thresholds/6.0/GRPCSwiftBenchmark.Metadata_Iterate_binary_values_when_only_strings_stored.p90.json

@@ -1,7 +1,3 @@
 {
-  "mallocCountTotal" : 2000,
-  "memoryLeaked" : 0,
-  "releaseCount" : 6001,
-  "retainCount" : 2000,
-  "syscalls" : 0
+  "mallocCountTotal": 1000
 }

IntegrationTests/Benchmarks/Thresholds/6.1/GRPCSwiftBenchmark.Metadata_Iterate_binary_values_when_only_strings_stored.p90.json

@@ -1,7 +1,3 @@
 {
-  "mallocCountTotal" : 2000,
-  "memoryLeaked" : 0,
-  "releaseCount" : 6001,
-  "retainCount" : 2000,
-  "syscalls" : 0
+  "mallocCountTotal": 1000
 }

IntegrationTests/Benchmarks/Thresholds/nightly-main/GRPCSwiftBenchmark.Metadata_Iterate_binary_values_when_only_strings_stored.p90.json

@@ -1,7 +1,3 @@
 {
-  "mallocCountTotal" : 2000,
-  "memoryLeaked" : 0,
-  "releaseCount" : 7001,
-  "retainCount" : 3000,
-  "syscalls" : 0
+  "mallocCountTotal": 1000
 }

IntegrationTests/Benchmarks/Thresholds/nightly-next/GRPCSwiftBenchmark.Metadata_Iterate_binary_values_when_only_strings_stored.p90.json

@@ -1,7 +1,3 @@
 {
-  "mallocCountTotal" : 2000,
-  "memoryLeaked" : 0,
-  "releaseCount" : 7001,
-  "retainCount" : 3000,
-  "syscalls" : 0
+  "mallocCountTotal": 1000
 }

Sources/GRPCCore/Metadata.swift

@@ -423,7 +423,7 @@ extension Metadata {
           switch value {
           case .string(let stringValue):
             do {
-              return try Base64.decode(string: stringValue)
+              return try Base64.decode(string: stringValue, options: [.omitPaddingCharacter])
             } catch {
               continue
             }

Tests/GRPCCoreTests/MetadataTests.swift

@@ -221,6 +221,18 @@ struct MetadataTests {
     #expect(Array(metadata[binaryValues: "key-bin"]) == expected)
   }
 
+  @Test("Iterate over unpadded base64 encoded binary values for a key")
+  @available(gRPCSwift 2.0, *)
+  func iterateOverUnpaddedBase64BinaryEncodedValuesForKey() {
+    let metadata: Metadata = [
+      "key-bin": "YQ==",
+      "key-bin": "YQ",
+    ]
+
+    let expected: [[UInt8]] = [[UInt8(ascii: "a")], [UInt8(ascii: "a")]]
+    #expect(Array(metadata[binaryValues: "key-bin"]) == expected)
+  }
+
   @Test("Subscripts are case-insensitive")
   @available(gRPCSwift 2.0, *)
   func subscriptIsCaseInsensitive() {