filters/tracing: improve stateBagToTag

[AlexanderYastrebov]

• reduce allocations when tag value is string
• check statebag value first to bail out early if it is absent
• restrict max number of arguments to two

goos: linux
goarch: amd64
pkg: github.com/zalando/skipper/filters/tracing
cpu: Intel(R) Core(TM) i5-8350U CPU @ 1.70GHz
                            │   HEAD~1    │                HEAD                 │
                            │   sec/op    │   sec/op     vs base                │
StateBagToTag_StringValue-8   313.1n ± 2%   124.3n ± 1%  -60.29% (p=0.000 n=10)

                            │   HEAD~1   │                HEAD                │
                            │    B/op    │   B/op     vs base                 │
StateBagToTag_StringValue-8   19.00 ± 0%   0.00 ± 0%  -100.00% (p=0.000 n=10)

                            │   HEAD~1   │                HEAD                 │
                            │ allocs/op  │ allocs/op   vs base                 │
StateBagToTag_StringValue-8   2.000 ± 0%   0.000 ± 0%  -100.00% (p=0.000 n=10)

For zalando-incubator/kubernetes-on-aws#6604

[AlexanderYastrebov]

See golang/go#64541 why not

if s, ok := value.(string); ok {
    span.SetTag(f.tagName, s)
}

[szuecs]

I guess we have similar cases all over the place.

[AlexanderYastrebov]

I checked all span.SetTag callsites and its fine. There may be other places that receive interface{} like SetTag of course but I think this is a very specific pattern here - optimizing for string case.

We could simply use span.SetTag(f.tagName, value) without type assertion but that is not guaranteed to work for types other than string, bool and ints:

// Adds a tag to the span.
	//
	// If there is a pre-existing tag set for `key`, it is overwritten.
	//
	// Tag values can be numeric types, strings, or bools. The behavior of
	// other tag value types is undefined at the OpenTracing level. If a
	// tracing system does not know how to handle a particular value type, it
	// may ignore the tag, but shall not panic.
	//
	// Returns a reference to this Span for chaining.
	SetTag(key string, value interface{}) Span

so I decided to keep fmt.Sprint()

[MustafaSaber]

👍

[AlexanderYastrebov]

👍