Add basic auth to prometheus input plugin (#6062)

plugins/inputs/prometheus/README.md

@@ -33,6 +33,11 @@ in Prometheus format.
   ## OR
   # bearer_token_string = "abc_123"
 
+  ## HTTP Basic Authentication username and password. ('bearer_token' and
+  ## 'bearer_token_string' take priority)
+  # username = ""
+  # password = ""
+
   ## Specify timeout duration for slower prometheus clients (default is 3s)
   # response_timeout = "3s"
 

plugins/inputs/prometheus/prometheus.go

@@ -34,6 +34,10 @@ type Prometheus struct {
 	BearerToken       string `toml:"bearer_token"`
 	BearerTokenString string `toml:"bearer_token_string"`
 
+	// Basic authentication credentials
+	Username string `toml:"username"`
+	Password string `toml:"password"`
+
 	ResponseTimeout internal.Duration `toml:"response_timeout"`
 
 	tls.ClientConfig
@@ -75,6 +79,11 @@ var sampleConfig = `
   ## OR
   # bearer_token_string = "abc_123"
 
+  ## HTTP Basic Authentication username and password. ('bearer_token' and
+  ## 'bearer_token_string' take priority)
+  # username = ""
+  # password = ""
+
 	## Specify timeout duration for slower prometheus clients (default is 3s)
   # response_timeout = "3s"
 
@@ -251,6 +260,8 @@ func (p *Prometheus) gatherURL(u URLAndAddress, acc telegraf.Accumulator) error
 		req.Header.Set("Authorization", "Bearer "+string(token))
 	} else if p.BearerTokenString != "" {
 		req.Header.Set("Authorization", "Bearer "+p.BearerTokenString)
+	} else if p.Username != "" || p.Password != "" {
+		req.SetBasicAuth(p.Username, p.Password)
 	}
 
 	var resp *http.Response