Implement Is() for multierrors

We need to be able to identify the errors that a multierror wraps.
Especially in case of `context.Canceled`. This is the same
implementation of that method as we do in github.com/grafana/dskit/multierror

Signed-off-by: Oleg Zaytsev <mail@olegzaytsev.com>

tsdb/errors/errors.go

@@ -16,6 +16,7 @@ package errors
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 	"io"
 )
@@ -79,6 +80,19 @@ func (es nonNilMultiError) Error() string {
 	return buf.String()
 }
 
+// Is attempts to match the provided error against errors in the error list.
+//
+// This function allows errors.Is to traverse the values stored in the MultiError.
+// It returns true if any of the errors in the list match the target.
+func (es nonNilMultiError) Is(target error) bool {
+	for _, err := range es.errs {
+		if errors.Is(err, target) {
+			return true
+		}
+	}
+	return false
+}
+
 // CloseAll closes all given closers while recording error in MultiError.
 func CloseAll(cs []io.Closer) error {
 	errs := NewMulti()