Bump contrib.go.opencensus.io/exporter/prometheus from 0.3.0 to 0.4.0

Bumps [contrib.go.opencensus.io/exporter/prometheus](https://github.com/census-ecosystem/opencensus-go-exporter-prometheus) from 0.3.0 to 0.4.0.
- [Release notes](https://github.com/census-ecosystem/opencensus-go-exporter-prometheus/releases)
- [Commits](census-ecosystem/opencensus-go-exporter-prometheus@v0.3.0...v0.4.0)

---
updated-dependencies:
- dependency-name: contrib.go.opencensus.io/exporter/prometheus
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

go.mod

@@ -1,7 +1,7 @@
 module github.com/slok/go-http-metrics
 
 require (
-	contrib.go.opencensus.io/exporter/prometheus v0.3.0
+	contrib.go.opencensus.io/exporter/prometheus v0.4.0
 	github.com/emicklei/go-restful v2.15.0+incompatible
 	github.com/fasthttp/router v1.4.1
 	github.com/gin-gonic/gin v1.7.4