Fix handling of the %z strptime format. (open-telemetry#29929)

**Description:**

`strptime(3)` says that `%z` is "an RFC-822/ISO 8601 standard timezone
specification", but the previous code did not allow the string "Z" to
signify UTC time, as required by ISO 8601.

**Link to tracking Issue:** none

**Testing:**

Added unit tests for both numeric and "Z" time zones.

**Documentation:** none

.chloggen/time-zulu.yaml

@@ -0,0 +1,33 @@
+# 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: time
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: The `%z` strptime format now correctly parses `Z` as a valid timezone
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues:
+- 29929
+
+# (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: |
+  `strptime(3)` says that `%z` is "an RFC-822/ISO 8601 standard
+  timezone specification", but the previous code did not allow the
+  string "Z" to signify UTC time, as required by ISO 8601. Now, both
+  `+0000` and `Z` are recognized as UTC times in all components that
+  handle `strptime` format strings.
+
+# 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]

internal/coreinternal/timeutils/internal/ctimefmt/ctimefmt.go

@@ -44,7 +44,7 @@ var ctimeSubstitutes = map[string]string{
 	"%f": "999999",
 	"%s": "99999999",
 	"%Z": "MST",
-	"%z": "-0700",
+	"%z": "Z0700",
 	"%w": "-070000",
 	"%i": "-07",
 	"%j": "-07:00",

internal/coreinternal/timeutils/internal/ctimefmt/ctimefmt_test.go

@@ -54,3 +54,25 @@ func TestParse(t *testing.T) {
 		t.Errorf("Given: %v, expected: %v", dt, dt2)
 	}
 }
+
+func TestZulu(t *testing.T) {
+	format := "%Y-%m-%dT%H:%M:%S.%L%z"
+	// These time should all parse as UTC.
+	for _, input := range []string{
+		"2019-01-02T15:04:05.666666Z",
+		"2019-01-02T15:04:05.666666-0000",
+		"2019-01-02T15:04:05.666666+0000",
+	} {
+		t.Run(input, func(t *testing.T) {
+			dt, err := Parse(format, input)
+			if err != nil {
+				t.Error(err)
+			} else if dt.UnixNano() != dt1.UnixNano() {
+				// We compare the unix nanoseconds because Go has a subtle parsing difference between "Z" and "+0000".
+				// The former returns a Time with the UTC timezone, the latter returns a Time with a 0000 time zone offset.
+				// (See Go's documentation for `time.Parse`.)
+				t.Errorf("Given: %v, expected: %v", dt, dt1)
+			}
+		})
+	}
+}