replace some fmt.Sprintfs with strconv

Teeny-tiny optimizations:

    BenchmarkSprintf-10       37735996    32.31  ns/op  0 B/op  0 allocs/op
    BenchmarkItoa-10         591945836     2.031 ns/op  0 B/op  0 allocs/op
    BenchmarkFormatUint-10   593701444     2.014 ns/op  0 B/op  0 allocs/op

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

integration/nri_test.go

@@ -602,7 +602,7 @@ func (tc *nriTest) setup() {
 		tc.prefix = strings.ToLower(tc.name)
 	}
 	if tc.namespace == "" {
-		tc.namespace = tc.prefix + "-" + fmt.Sprintf("%d", os.Getpid())
+		tc.namespace = tc.prefix + "-" + strconv.Itoa(os.Getpid())
 	}
 
 	tc.sbCfg = make(map[string]*runtime.PodSandboxConfig)