update branch names in a few links (#1039)

Signed-off-by: Christoph Mewes <[email protected]>

Author: xrstf

.gitignore

@@ -11,7 +11,7 @@ examples/random/random
 vendor/
 
 # The remainder of this file is taken from
-# https://github.com/github/gitignore/blob/master/Go.gitignore
+# https://github.com/github/gitignore/blob/main/Go.gitignore
 
 # Binaries for programs and plugins
 *.exe

CODE_OF_CONDUCT.md

@@ -1,3 +1,3 @@
 ## Prometheus Community Code of Conduct
 
-Prometheus follows the [CNCF Code of Conduct](https://github.com/cncf/foundation/blob/master/code-of-conduct.md).
+Prometheus follows the [CNCF Code of Conduct](https://github.com/cncf/foundation/blob/main/code-of-conduct.md).

README.md

@@ -1,6 +1,6 @@
 # Prometheus Go client library
 
-[![CircleCI](https://circleci.com/gh/prometheus/client_golang/tree/master.svg?style=svg)](https://circleci.com/gh/prometheus/client_golang/tree/master)
+[![CircleCI](https://circleci.com/gh/prometheus/client_golang/tree/main.svg?style=svg)](https://circleci.com/gh/prometheus/client_golang/tree/main)
 [![Go Report Card](https://goreportcard.com/badge/github.com/prometheus/client_golang)](https://goreportcard.com/report/github.com/prometheus/client_golang)
 [![Go Reference](https://pkg.go.dev/badge/github.com/prometheus/client_golang.svg)](https://pkg.go.dev/github.com/prometheus/client_golang)
 [![Slack](https://img.shields.io/badge/join%20slack-%23prometheus--client_golang-brightgreen.svg)](https://slack.cncf.io/)
@@ -35,21 +35,21 @@ coexist for a while to enable a convenient transition.
 [![code-coverage](http://gocover.io/_badge/github.com/prometheus/client_golang/prometheus)](http://gocover.io/github.com/prometheus/client_golang/prometheus) [![Go Reference](https://pkg.go.dev/badge/github.com/prometheus/client_golang/prometheus.svg)](https://pkg.go.dev/github.com/prometheus/client_golang/prometheus)
 
 The
-[`prometheus` directory](https://github.com/prometheus/client_golang/tree/master/prometheus)
+[`prometheus` directory](https://github.com/prometheus/client_golang/tree/main/prometheus)
 contains the instrumentation library. See the
 [guide](https://prometheus.io/docs/guides/go-application/) on the Prometheus
 website to learn more about instrumenting applications.
 
 The
-[`examples` directory](https://github.com/prometheus/client_golang/tree/master/examples)
+[`examples` directory](https://github.com/prometheus/client_golang/tree/main/examples)
 contains simple examples of instrumented code.
 
 ## Client for the Prometheus HTTP API
 
 [![code-coverage](http://gocover.io/_badge/github.com/prometheus/client_golang/api/prometheus/v1)](http://gocover.io/github.com/prometheus/client_golang/api/prometheus/v1) [![Go Reference](https://pkg.go.dev/badge/github.com/prometheus/client_golang/api.svg)](https://pkg.go.dev/github.com/prometheus/client_golang/api)
 
 The
-[`api/prometheus` directory](https://github.com/prometheus/client_golang/tree/master/api/prometheus)
+[`api/prometheus` directory](https://github.com/prometheus/client_golang/tree/main/api/prometheus)
 contains the client for the
 [Prometheus HTTP API](http://prometheus.io/docs/querying/api/). It allows you
 to write Go applications that query time series data from a Prometheus
@@ -58,14 +58,14 @@ server. It is still in alpha stage.
 ## Where is `model`, `extraction`, and `text`?
 
 The `model` packages has been moved to
-[`prometheus/common/model`](https://github.com/prometheus/common/tree/master/model).
+[`prometheus/common/model`](https://github.com/prometheus/common/tree/main/model).
 
 The `extraction` and `text` packages are now contained in
-[`prometheus/common/expfmt`](https://github.com/prometheus/common/tree/master/expfmt).
+[`prometheus/common/expfmt`](https://github.com/prometheus/common/tree/main/expfmt).
 
 ## Contributing and community
 
 See the [contributing guidelines](CONTRIBUTING.md) and the
 [Community section](http://prometheus.io/community/) of the homepage.
 
-`clint_golang` community is also present on the CNCF Slack `#prometheus-client_golang`.  
+`clint_golang` community is also present on the CNCF Slack `#prometheus-client_golang`.