Consolidate AWS credentials (#1208)

* Use shared AWS credential configuration.

*  Cloudwatch dimension wilcards 

* Allow configuring cache_ttl for cloudwatch metrics.

* Allow for wildcard in dimension values to select all available metrics.

* Use internal.Duration for CacheTTL and go fmt

* Refactor to not use embedded structs for config.

* Update AWS plugin READMEs with credentials details, update Changelog.

* Fix changelog after rebasing to master and 0.13.1 release.

* Fix changelog after rebase.

internal/config/aws/credentials.go

@@ -0,0 +1,49 @@
+package aws
+
+import (
+	"github.com/aws/aws-sdk-go/aws"
+	"github.com/aws/aws-sdk-go/aws/client"
+	"github.com/aws/aws-sdk-go/aws/credentials"
+	"github.com/aws/aws-sdk-go/aws/credentials/stscreds"
+	"github.com/aws/aws-sdk-go/aws/session"
+)
+
+type CredentialConfig struct {
+	Region    string
+	AccessKey string
+	SecretKey string
+	RoleARN   string
+	Profile   string
+	Filename  string
+	Token     string
+}
+
+func (c *CredentialConfig) Credentials() client.ConfigProvider {
+	if c.RoleARN != "" {
+		return c.assumeCredentials()
+	} else {
+		return c.rootCredentials()
+	}
+}
+
+func (c *CredentialConfig) rootCredentials() client.ConfigProvider {
+	config := &aws.Config{
+		Region: aws.String(c.Region),
+	}
+	if c.AccessKey != "" || c.SecretKey != "" {
+		config.Credentials = credentials.NewStaticCredentials(c.AccessKey, c.SecretKey, c.Token)
+	} else if c.Profile != "" || c.Filename != "" {
+		config.Credentials = credentials.NewSharedCredentials(c.Filename, c.Profile)
+	}
+
+	return session.New(config)
+}
+
+func (c *CredentialConfig) assumeCredentials() client.ConfigProvider {
+	rootCredentials := c.rootCredentials()
+	config := &aws.Config{
+		Region: aws.String(c.Region),
+	}
+	config.Credentials = stscreds.NewCredentials(rootCredentials, c.RoleARN)
+	return session.New(config)
+}