Don't bother with checking for go-kit/log stack frames until we've se…

…en SpanLogger

This makes the cost of spanlogger.Caller negligible for use outside of
SpanLogger:

                            │ SpanLoggerAwareCaller/with_go-kit's_Caller-10 │ SpanLoggerAwareCaller/with_dskit's_spanlogger.Caller-10 │
                            │                    sec/op                     │                 sec/op                  vs base         │
comparison-after-step-2.txt                                     927.5n ± 5%                              932.1n ± 2%  ~ (p=0.093 n=6)

                            │ SpanLoggerAwareCaller/with_go-kit's_Caller-10 │ SpanLoggerAwareCaller/with_dskit's_spanlogger.Caller-10 │
                            │                     B/op                      │                  B/op                   vs base         │
comparison-after-step-2.txt                                      732.5 ± 0%                               733.0 ± 0%  ~ (p=0.636 n=6)

                            │ SpanLoggerAwareCaller/with_go-kit's_Caller-10 │ SpanLoggerAwareCaller/with_dskit's_spanlogger.Caller-10 │
                            │                   allocs/op                   │              allocs/op                vs base           │
comparison-after-step-2.txt                                      9.000 ± 0%                             9.000 ± 0%  ~ (p=1.000 n=6) ¹
¹ all samples are equal

spanlogger/spanlogger.go

@@ -198,6 +198,7 @@ func (s *SpanLogger) SetSpanAndLogTag(key string, value interface{}) {
 func Caller(defaultStackDepth int) log.Valuer {
 	return func() interface{} {
 		stackDepth := defaultStackDepth
+		seenSpanLogger := false
 
 		for {
 			_, file, line, ok := runtime.Caller(stackDepth)
@@ -206,12 +207,22 @@ func Caller(defaultStackDepth int) log.Valuer {
 				return "<unknown>"
 			}
 
-			// If we're in this file, or log.go in the go-kit/log package, we need to continue searching in the stack.
-			//
+			// If we're in this file, or log.go in the go-kit/log package after having seen this file, we need to continue searching in the stack.
+			if strings.HasSuffix(file, "dskit/spanlogger/spanlogger.go") {
+				seenSpanLogger = true
+				stackDepth++
+				continue
+			}
+
 			// The path to log.go varies depending on how the package is imported:
 			// - if go-kit/log is vendored, the path is something like <project root>/vendor/github.com/go-kit/log/log.go
 			// - if it is not vendored, the path is something like /home/user/go/pkg/mod/github.com/go-kit/[email protected]/log.go and varies based on the version imported
-			if strings.HasSuffix(file, "dskit/spanlogger/spanlogger.go") || (strings.HasSuffix(file, "/log.go") && strings.Contains(file, "github.com/go-kit/log")) {
+			//
+			// We need to check for go-kit/log stack frames because log.With, log.WithPrefix or log.WithSuffix (including the various level methods like level.Debug,
+			// level.Info etc.) to wrap a SpanLogger introduces an additional context.Log stack frame that calls into the SpanLogger. This is because the use of SpanLogger
+			// as the logger means the optimisation to avoid creating a new logger in https://github.com/go-kit/log/blob/c7bf81493e581feca11e11a7672b14be3591ca43/log.go#L141-L146
+			// used by those methods can't be used, and so the SpanLogger is wrapped in a new logger.
+			if seenSpanLogger && (strings.HasSuffix(file, "/log.go") && strings.Contains(file, "github.com/go-kit/log")) {
 				stackDepth++
 				continue
 			}