fix: TestCreateCollectorProxy unit test failing on go-tip (#6204)

## Which problem is this PR solving?
- Resolves #6198 

## Description of the changes
- After the addition of `t.Parallel` in `cmd/agent/app/builder_test.go`
in `TestCreateCollectorProxy`, its test started to fail during random
ittterations. This was because in
`/cmd/agent/app/reporter/connect_metrics.go` under `NewConnectMetrics`
function we are doing `expvar.NewString("gRPCTarget")` which isnt
thread-safe and it should not be changed on initialized
- Now we use sync.Once to tackle the above bug/problem

## How was this change tested?
- running `go test -v`

## Checklist
- [x] I have read
https://github.com/jaegertracing/jaeger/blob/master/CONTRIBUTING_GUIDELINES.md
- [x] I have signed all commits
- [ ] I have added unit tests for the new functionality
- [x] I have run lint and test steps successfully
  - for `jaeger`: `make lint test`
  - for `jaeger-ui`: `yarn lint` and `yarn test`

Signed-off-by: Saumya Shah <[email protected]>
Co-authored-by: Yuri Shkuro <[email protected]>

cmd/agent/app/reporter/connect_metrics.go

@@ -5,6 +5,7 @@ package reporter
 
 import (
 	"expvar"
+	"sync"
 
 	"github.com/jaegertracing/jaeger/pkg/metrics"
 )
@@ -23,17 +24,21 @@ type ConnectMetrics struct {
 	target  *expvar.String
 }
 
+var (
+	gRPCTarget     *expvar.String
+	gRPCTargetOnce sync.Once
+)
+
 // NewConnectMetrics will be initialize ConnectMetrics
 func NewConnectMetrics(mf metrics.Factory) *ConnectMetrics {
 	cm := &ConnectMetrics{}
 	metrics.MustInit(&cm.metrics, mf.Namespace(metrics.NSOptions{Name: "connection_status"}), nil)
 
-	if r := expvar.Get("gRPCTarget"); r == nil {
-		cm.target = expvar.NewString("gRPCTarget")
-	} else {
-		cm.target = r.(*expvar.String)
-	}
-
+	// expvar.String is not thread-safe, so we need to initialize it only once
+	gRPCTargetOnce.Do(func() {
+		gRPCTarget = expvar.NewString("gRPCTarget")
+	})
+	cm.target = gRPCTarget
 	return cm
 }