go.mod: bump go-immutable-radix to v1.3.1

Fixes errors in tooling that relies on the existing `go.mod` file:

```
pkg/darwin_arm64/github.com/armon/go-metrics/metrics.go, line 8, column 2: could not import github.com/hashicorp/go-immutable-radix (open : no such file or directory)
pkg/darwin_arm64/github.com/armon/go-metrics/metrics.go, line 166, column 13: undefined: iradix
pkg/darwin_arm64/github.com/armon/go-metrics/start.go, line 37, column 17: undefined: iradix
```

Author: sean-

go.mod

@@ -3,16 +3,16 @@ module github.com/hashicorp/go-metrics
 go 1.12
 
 require (
-	github.com/DataDog/datadog-go v3.2.0+incompatible
+	github.com/DataDog/datadog-go v4.8.3+incompatible
 	github.com/circonus-labs/circonus-gometrics v2.3.1+incompatible
 	github.com/circonus-labs/circonusllhist v0.1.3 // indirect
-	github.com/golang/protobuf v1.4.3
+	github.com/golang/protobuf v1.5.4
 	github.com/hashicorp/go-immutable-radix v1.3.1
 	github.com/hashicorp/go-retryablehttp v0.5.3 // indirect
-	github.com/pascaldekloe/goe v0.1.0
-	github.com/prometheus/client_golang v1.11.1
-	github.com/prometheus/client_model v0.2.0
-	github.com/prometheus/common v0.26.0
+	github.com/pascaldekloe/goe v0.1.1
+	github.com/prometheus/client_golang v1.19.1
+	github.com/prometheus/client_model v0.6.1
+	github.com/prometheus/common v0.55.0
 	github.com/tv42/httpunix v0.0.0-20150427012821-b75d8614f926 // indirect
 )