fix(fielder): eliminate expensive inner calls (#33)

## Which problem is this PR solving?

- Fielder is much more expensive than it should be; this only turned up
from profiling a real output, not a dummy output, since Fielder does not
get invoked except on real output.

![image
(25)](https://github.com/honeycombio/loadgen/assets/614704/ec3e7877-821e-45fa-98fb-89635a6fee28)


## Short description of the changes

- Move regexp.MustCompile out of the hot path
- Eliminate getGoroutineId since it requires gathering a full traceback
which is very slow.

---------

Co-authored-by: Kent Quirk <kentquirk@honeycomb.io>

fielder.go

@@ -6,7 +6,6 @@ import (
 	"math/rand"
 	"os"
 	"regexp"
-	"runtime"
 	"strconv"
 	"strings"
 
@@ -52,6 +51,10 @@ var nouns = []string{
 	"watch", "wheel", "whip", "whistle", "window", "wing", "wire", "worm",
 }
 
+var constpat = regexp.MustCompile(`^([a-zA-Z0-9_]+)=([^/].*)$`)
+var genpat = regexp.MustCompile(`^((?:[0-9]+\.)?[a-zA-Z0-9_]+)=/([ibfsu][awxrgqt]?)([0-9.-]+)?(,[0-9.-]+)?$`)
+var keypat = regexp.MustCompile(`^([0-9]+)\.(.*$)`)
+
 type Rng struct {
 	rng *rand.Rand
 }
@@ -122,33 +125,6 @@ func (r Rng) BoolWithProb(p int) bool {
 	return r.Int(0, 100) < int64(p)
 }
 
-func getGoroutineID() uint64 {
-	var buffer [31]byte
-	written := runtime.Stack(buffer[:], false)
-	index := 10
-	negative := buffer[index] == '-'
-	if negative {
-		index = 11
-	}
-	id := uint64(0)
-	for index < written {
-		byte := buffer[index]
-		if byte == ' ' {
-			break
-		}
-		if byte < '0' || byte > '9' {
-			panic("could not get goroutine ID")
-		}
-		id *= 10
-		id += uint64(byte - '0')
-		index++
-	}
-	if negative {
-		id = -id
-	}
-	return id
-}
-
 // getProcessID returns the process ID
 func getProcessID() int64 {
 	return int64(os.Getpid())
@@ -191,8 +167,6 @@ func getWordList(rng Rng, cardinality int, source []string) []string {
 // parseUserFields expects a list of fields in the form of name=constant or name=/gen.
 // See README.md for more information.
 func parseUserFields(rng Rng, userfields []string) (map[string]func() any, error) {
-	constpat := regexp.MustCompile(`^([a-zA-Z0-9_]+)=([^/].*)$`)
-	genpat := regexp.MustCompile(`^((?:[0-9]+\.)?[a-zA-Z0-9_]+)=/([ibfsu][awxrgqt]?)([0-9.-]+)?(,[0-9.-]+)?$`)
 	// groups                                        1                   2	         3         4
 	fields := make(map[string]func() any)
 	for _, field := range userfields {
@@ -436,7 +410,7 @@ type Fielder struct {
 // service names to generate. The field names are randomly generated by
 // combining an adjective and a noun and are consistent for a given fielder.
 // The field values are randomly generated.
-// Fielder also includes two special fields: goroutine_id and process_id.
+// Fielder also includes the process_id.
 func NewFielder(seed string, userFields []string, nextras, nservices int) (*Fielder, error) {
 	rng := NewRng(seed)
 	gens := rng.getValueGenerators()
@@ -448,7 +422,6 @@ func NewFielder(seed string, userFields []string, nextras, nservices int) (*Fiel
 		fieldname := rng.WordPair()
 		fields[fieldname] = gens[rng.Intn(len(gens))]
 	}
-	fields["goroutine_id"] = func() any { return getGoroutineID() }
 	fields["process_id"] = func() any { return getProcessID() }
 
 	names := make([]string, nservices)
@@ -467,7 +440,6 @@ func (f *Fielder) GetServiceName(n int) string {
 // indicate that the field should be included at a specific
 // level in the trace, where 0 is the root.
 func (f *Fielder) atLevel(name string, level int) (string, bool) {
-	keypat := regexp.MustCompile(`^([0-9]+)\.(.*$)`)
 	matches := keypat.FindStringSubmatch(name)
 	if len(matches) == 0 {
 		return name, true