Use string([]byte) as a cache key in decoder

Go has a special case for `string([]byte)` that doesn't allocate:

* golang/go@f5f5a8b6209f8

Before and after:

```
BenchmarkCache/cached-10   132218238   9.292 ns/op       0 B/op       0 allocs/op
BenchmarkCache/cached-10   212642139   5.694 ns/op       0 B/op       0 allocs/op
```

Author: bobrik

decoder/decoder.go

@@ -1,10 +1,8 @@
 package decoder
 
 import (
-	"bytes"
 	"errors"
 	"fmt"
-	"hash/maphash"
 	"sync"
 
 	"github.com/cloudflare/ebpf_exporter/v2/config"
@@ -14,11 +12,6 @@ import (
 // ErrSkipLabelSet instructs exporter to skip label set
 var ErrSkipLabelSet = errors.New("this label set should be skipped")
 
-type cacheValue struct {
-	in  []byte
-	out []string
-}
-
 // Decoder transforms byte field value into a byte value representing string
 // to either use as an input for another Decoder or to use as the final
 // label value for Prometheus metrics
@@ -30,8 +23,7 @@ type Decoder interface {
 type Set struct {
 	mu       sync.Mutex
 	decoders map[string]Decoder
-	cache    map[uint64]cacheValue
-	seed     maphash.Seed
+	cache    map[string][]string
 }
 
 // NewSet creates a Set with all known decoders
@@ -65,8 +57,7 @@ func NewSet() (*Set, error) {
 			"syscall":      &Syscall{},
 			"uint":         &UInt{},
 		},
-		cache: map[uint64]cacheValue{},
-		seed:  maphash.MakeSeed(),
+		cache: map[string][]string{},
 	}, nil
 }
 
@@ -98,20 +89,18 @@ func (s *Set) Decode(in []byte, label config.Label) ([]byte, error) {
 // DecodeLabels transforms eBPF map key bytes into a list of label values
 // according to configuration
 func (s *Set) DecodeLabels(in []byte, labels []config.Label) ([]string, error) {
-	cacheKey := maphash.Bytes(s.seed, in)
-	if cached, ok := s.cache[cacheKey]; ok {
-		// Make sure that there is no collision here
-		if bytes.Equal(cached.in, in) {
-			return cached.out, nil
-		}
+	// string(in) must not be a variable to avoid allocation:
+	// * https://github.com/golang/go/commit/f5f5a8b6209f8
+	if cached, ok := s.cache[string(in)]; ok {
+		return cached, nil
 	}
 
 	values, err := s.decodeLabels(in, labels)
 	if err != nil {
 		return nil, err
 	}
 
-	s.cache[cacheKey] = cacheValue{in: in, out: values}
+	s.cache[string(in)] = values
 
 	return values, nil
 }