support go1.15+

Dependencies of this module no longer support go versions < 1.15 so
a recent update caused these tests to fail:

https://github.com/googleapis/google-cloud-go#go-versions-supported

Update this module to drop support for 1.12-1.14.

Author: tam7t

.github/workflows/go.yml

@@ -13,10 +13,10 @@ jobs:
       fail-fast: false
       matrix:
         go: [
-          '1.12',
-          '1.13',
-          '1.14',
-          '1.15'
+          '1.15',
+          '1.16',
+          '1.17',
+          '1.18'
         ]
 
     name: unit

example/main.go

@@ -15,6 +15,7 @@ package main
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"log"
 	"math/rand"
@@ -79,7 +80,7 @@ func main() {
 		Addr: ":" + port,
 	}
 	go func() {
-		if err := srv.ListenAndServe(); err != nil {
+		if err := srv.ListenAndServe(); err != nil && !errors.Is(err, http.ErrServerClosed) {
 			log.Printf("server error: %s", err)
 		}
 	}()

go.mod

@@ -1,6 +1,6 @@
 module github.com/google/go-metrics-stackdriver
 
-go 1.12
+go 1.15
 
 require (
 	cloud.google.com/go v0.101.1 // indirect