[pkg/ottl] Replace key when using function (open-telemetry#38626)

#### Description

When using the `replace_all_patterns` with mode set to `"key"` like
`replace_all_patterns(cache, "key", "^(.*)$", "$$1", ToSnakeCase)`. This
caused the value to be replaced instead of the key. By replacing the
statement updating the value it now work as expected. At least this is
how I interpret this function to work.

#### Link to tracking issue

open-telemetry#32896

#### Testing

Added unit test for this scenario since it was missing.

#### Documentation

---------

Signed-off-by: Patrik Nyström <[email protected]>

Author: panys9

.chloggen/fix-ottl-replace-all-patterns.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: bug_fix
+
+# 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: "Fix so replace_all_patterns can replace keys using optional function"
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [32896]
+
+# (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: When using the `replace_all_patterns` with `key` and `optional` function on the replacement, the value was being replaced with the key. This change fixes that and now the key is replaced as intended.
+
+# 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: []

pkg/ottl/ottlfuncs/func_replace_all_patterns.go

@@ -83,11 +83,11 @@ func replaceAllPatterns[K any](target ottl.PMapGetter[K], mode string, regexPatt
 			case modeKey:
 				if compiledPattern.MatchString(key) {
 					if !fn.IsEmpty() {
-						updatedString, err := applyOptReplaceFunction(ctx, tCtx, compiledPattern, fn, key, replacementVal, replacementFormat)
+						updatedKey, err := applyOptReplaceFunction(ctx, tCtx, compiledPattern, fn, key, replacementVal, replacementFormat)
 						if err != nil {
 							break AttributeLoop
 						}
-						updated.PutStr(key, updatedString)
+						originalValue.CopyTo(updated.PutEmpty(updatedKey))
 					} else {
 						updatedKey := compiledPattern.ReplaceAllString(key, replacementVal)
 						originalValue.CopyTo(updated.PutEmpty(updatedKey))

pkg/ottl/ottlfuncs/func_replace_all_patterns_test.go

@@ -514,6 +514,28 @@ func Test_replaceAllPatterns(t *testing.T) {
 				expectedMap.PutStr("test7", "empty_string_replacement")
 			},
 		},
+		{
+			name:    "replacement matches with function",
+			target:  target,
+			mode:    modeKey,
+			pattern: `test(\d)`,
+			replacement: ottl.StandardStringGetter[pcommon.Map]{
+				Getter: func(context.Context, pcommon.Map) (any, error) {
+					return "$1", nil
+				},
+			},
+			replacementFormat: ottl.Optional[ottl.StringGetter[pcommon.Map]]{},
+			function:          optionalArg,
+			want: func(expectedMap pcommon.Map) {
+				expectedMap.PutStr("test", "hello world")
+				expectedMap.PutStr("hash(2)", "hello")
+				expectedMap.PutStr("hash(3)", "goodbye world1 and world2")
+				expectedMap.PutInt("hash(4)", 1234)
+				expectedMap.PutDouble("hash(5)", 1234)
+				expectedMap.PutBool("hash(6)", true)
+				expectedMap.PutStr("hash(7)", "")
+			},
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {