perf: reduce allocation in MemoizeFields for common cases (#1726)

## Which problem is this PR solving?

All key fields are memoized during ingest, `MemoizeFields` are called
right before making a sampling decision. Its purpose is to make sure key
fields are memoized once even after a config reload. Config reload
should be rare. We don't need to pre-allocate the `keysToFind` slice
during normal operation.

## Short description of the changes

- don't pre-allocate `keysToFind` for common cases

Author: VinozzZ

types/payload.go

@@ -568,7 +568,14 @@ func (p *Payload) MemoizeFields(keys ...string) {
 		p.missingFields = make(map[string]struct{}, len(keys))
 	}
 
-	keysToFind := make(map[string]struct{}, len(keys))
+	// It is rare for a key field to not be memoized.
+	// Intentionally not allocating memory for keysToFind because it is rarely needed.
+	// It is worth the compute cost to grow this map on those rare occassions instead
+	// of allocating memory we rarely use.
+	// CAUTION: This optimization is under the assumption that MemoizeFields() are only
+	// called after the first memoization operation. If this assumption ever changes,
+	// we should reevaluate this optimization
+	keysToFind := make(map[string]struct{})
 	for _, key := range keys {
 		if _, ok := p.missingFields[key]; ok {
 			continue