MM-23568: Add rudder to server diagnostics. (#14151)

* MM-23568: Add rudder to server diagnostics.

* Add unit test.

* Go mod tidy.

* CSP Header fix.

* Fix review comments.

* Update web/handlers.go

Co-Authored-By: Jesse Hallam <jesse.hallam@gmail.com>

* Partially address review comments.

* fix tests.

* Finish implementing review suggestions and then fixing tests.

* Fix CSP Header tests.

Co-authored-by: Jesse Hallam <jesse.hallam@gmail.com>

vendor/github.com/rudderlabs/analytics-go/error.go

@@ -0,0 +1,60 @@
+package analytics
+
+import (
+	"errors"
+	"fmt"
+)
+
+// Returned by the `NewWithConfig` function when the one of the configuration
+// fields was set to an impossible value (like a negative duration).
+type ConfigError struct {
+
+	// A human-readable message explaining why the configuration field's value
+	// is invalid.
+	Reason string
+
+	// The name of the configuration field that was carrying an invalid value.
+	Field string
+
+	// The value of the configuration field that caused the error.
+	Value interface{}
+}
+
+func (e ConfigError) Error() string {
+	return fmt.Sprintf("analytics.NewWithConfig: %s (analytics.Config.%s: %#v)", e.Reason, e.Field, e.Value)
+}
+
+// Instances of this type are used to represent errors returned when a field was
+// no initialize properly in a structure passed as argument to one of the
+// functions of this package.
+type FieldError struct {
+
+	// The human-readable representation of the type of structure that wasn't
+	// initialized properly.
+	Type string
+
+	// The name of the field that wasn't properly initialized.
+	Name string
+
+	// The value of the field that wasn't properly initialized.
+	Value interface{}
+}
+
+func (e FieldError) Error() string {
+	return fmt.Sprintf("%s.%s: invalid field value: %#v", e.Type, e.Name, e.Value)
+}
+
+var (
+	// This error is returned by methods of the `Client` interface when they are
+	// called after the client was already closed.
+	ErrClosed = errors.New("the client was already closed")
+
+	// This error is used to notify the application that too many requests are
+	// already being sent and no more messages can be accepted.
+	ErrTooManyRequests = errors.New("too many requests are already in-flight")
+
+	// This error is used to notify the client callbacks that a message send
+	// failed because the JSON representation of a message exceeded the upper
+	// limit.
+	ErrMessageTooBig = errors.New("the message exceeds the maximum allowed size")
+)