Enhance the Decode OTTL function to support all flavors of Base64 (open-telemetry#38854)

#### Description

Adding a feature - Enhance the Decode OTTL function to support all
flavors of Base64

#### Testing

Added tests and ran all tests for the extension.

#### Documentation

Updated the OTTL readme

---------

Co-authored-by: Edmo Vamerlatti Costa <[email protected]>

Author: nahguam

.chloggen/ottl-decode-base64.yaml

@@ -0,0 +1,27 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: pkg/ottl
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Enhance the Decode OTTL function to support all flavors of Base64
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [38854]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: [user]

pkg/ottl/ottlfuncs/README.md

@@ -554,7 +554,7 @@ Examples:
 The `Decode` Converter takes a string or byte array encoded with the specified encoding and returns the decoded string.
 
 `value` is a valid encoded string or byte array.
-`encoding` is a valid encoding name included in the [IANA encoding index](https://www.iana.org/assignments/character-sets/character-sets.xhtml).
+`encoding` is a valid encoding name included in the [IANA encoding index](https://www.iana.org/assignments/character-sets/character-sets.xhtml) or one of `base64`, `base64-raw`, `base64-url` or `base64-raw-url`.
 
 Examples:
 

pkg/ottl/ottlfuncs/func_decode.go

@@ -60,13 +60,15 @@ func Decode[K any](target ottl.Getter[K], encoding string) (ottl.ExprFunc[K], er
 		}
 
 		switch encoding {
+		// base64 is not in IANA index, so we have to deal with this encoding separately
 		case "base64":
-			// base64 is not in IANA index, so we have to deal with this encoding separately
-			decodedBytes, err := base64.StdEncoding.DecodeString(stringValue)
-			if err != nil {
-				return nil, fmt.Errorf("could not decode: %w", err)
-			}
-			return string(decodedBytes), nil
+			return decodeBase64(base64.StdEncoding, stringValue)
+		case "base64-raw":
+			return decodeBase64(base64.RawStdEncoding, stringValue)
+		case "base64-url":
+			return decodeBase64(base64.URLEncoding, stringValue)
+		case "base64-raw-url":
+			return decodeBase64(base64.RawURLEncoding, stringValue)
 		default:
 			e, err := textutils.LookupEncoding(encoding)
 			if err != nil {
@@ -82,3 +84,11 @@ func Decode[K any](target ottl.Getter[K], encoding string) (ottl.ExprFunc[K], er
 		}
 	}, nil
 }
+
+func decodeBase64(encoding *base64.Encoding, stringValue string) (any, error) {
+	decodedBytes, err := encoding.DecodeString(stringValue)
+	if err != nil {
+		return nil, fmt.Errorf("could not decode: %w", err)
+	}
+	return string(decodedBytes), nil
+}

pkg/ottl/ottlfuncs/func_decode_test.go

@@ -171,6 +171,30 @@ func TestDecode(t *testing.T) {
 			encoding:      "base64",
 			expectedError: "illegal base64 data at input byte",
 		},
+		{
+			name:     "base64 with url-safe sensitive characters",
+			value:    "R28/L1p+eA==",
+			encoding: "base64",
+			want:     "Go?/Z~x",
+		},
+		{
+			name:     "base64-raw with url-safe sensitive characters",
+			value:    "R28/L1p+eA",
+			encoding: "base64-raw",
+			want:     "Go?/Z~x",
+		},
+		{
+			name:     "base64-url with url-safe sensitive characters",
+			value:    "R28_L1p-eA==",
+			encoding: "base64-url",
+			want:     "Go?/Z~x",
+		},
+		{
+			name:     "base64-raw-url with url-safe sensitive characters",
+			value:    "R28_L1p-eA",
+			encoding: "base64-raw-url",
+			want:     "Go?/Z~x",
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {