Add deprecate warning

Author: constanca-m

receiver/azuremonitorreceiver/README.md

@@ -24,7 +24,7 @@ The following settings are required:
 The following settings are optional:
 
 - `credentials` to get valid credentials for azure. It should be the id of an `azureauthextension` component.
--  `auth`, if `credentials` are not specified (default = service_principal): Specifies the used authentication method:
+- `auth` (deprecated), if `credentials` are not specified (default = service_principal). Specifies the used authentication method:
   - Supported values for `auth` are `service_principal`, `workload_identity`, `managed_identity`, `default_credentials`.
 - `resource_groups` (default = none): Filter metrics for specific resource groups, not setting a value will scrape metrics for all resources in the subscription.
 - `services` (default = none): Filter metrics for specific services, not setting a value will scrape metrics for all services integrated with Azure Monitor.

receiver/azuremonitorreceiver/config.go

@@ -259,6 +259,7 @@ type Config struct {
 	// useless.
 	Credentials string `mapstructure:"credentials"`
 
+	// Authentication is deprecated.
 	Authentication     string `mapstructure:"auth"`
 	TenantID           string `mapstructure:"tenant_id"`
 	ClientID           string `mapstructure:"client_id"`

receiver/azuremonitorreceiver/scraper.go

@@ -186,6 +186,7 @@ func (s *azureScraper) loadCredentials(host component.Host) (err error) {
 		return nil
 	}
 
+	s.settings.Logger.Warn("'auth' is deprecated, use 'credentials' instead")
 	switch s.cfg.Authentication {
 	case defaultCredentials:
 		if s.cred, err = s.azDefaultCredentialsFunc(nil); err != nil {