Feature: Allow getting credentials via EC2 role (#3343)

* Allow getting credentials via EC2 role

This is subtly different than the existing `role_arn` solution, which
allows Prometheus to assume an IAM role given some set of credentials
already in-scope. With EC2 roles, one specifies the role at instance
launch time (via an instance profile.) The instance then exposes
temporary credentials via its metadata. The AWS Go SDK exposes a
credential provider that polls the [instance metadata endpoint][1]
already, so we can simply use that and it will take care of renewing the
credentials when they expire.

Without this, if this is being used inside EC2, it is difficult to
cleanly allow the use of STS credentials. One has to set up a proxy role
that can assume the role you really want, and launch the EC2 instance
with the proxy role. This isn't very clean, and also doesn't seem to be
[supported very well][2].

[1]:
http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-instance-metadata.html
[2]: https://github.com/aws/aws-cli/issues/1390

* Automatically try to detect EC2 role credentials

The `Available()` function exposed on ec2metadata returns a simple
true/false if the ec2 metadata is available. This is the best way to
know if we're actually running in EC2 (which is the only valid use-case
for this credential provider.)

This allows this to "just work" if you are using EC2 instance roles.

discovery/ec2/ec2.go

@@ -22,7 +22,9 @@ import (
 
 	"github.com/aws/aws-sdk-go/aws"
 	"github.com/aws/aws-sdk-go/aws/credentials"
+	"github.com/aws/aws-sdk-go/aws/credentials/ec2rolecreds"
 	"github.com/aws/aws-sdk-go/aws/credentials/stscreds"
+	"github.com/aws/aws-sdk-go/aws/ec2metadata"
 	"github.com/aws/aws-sdk-go/aws/session"
 	"github.com/go-kit/kit/log"
 	"github.com/go-kit/kit/log/level"
@@ -137,6 +139,16 @@ func (d *Discovery) Run(ctx context.Context, ch chan<- []*config.TargetGroup) {
 	}
 }
 
+func (d *Discovery) ec2MetadataAvailable(sess *session.Session) (isAvailable bool) {
+	svc := ec2metadata.New(sess, &aws.Config{
+		MaxRetries: aws.Int(0),
+	})
+
+	isAvailable = svc.Available()
+
+	return isAvailable
+}
+
 func (d *Discovery) refresh() (tg *config.TargetGroup, err error) {
 	t0 := time.Now()
 	defer func() {
@@ -159,7 +171,12 @@ func (d *Discovery) refresh() (tg *config.TargetGroup, err error) {
 		creds := stscreds.NewCredentials(sess, d.roleARN)
 		ec2s = ec2.New(sess, &aws.Config{Credentials: creds})
 	} else {
-		ec2s = ec2.New(sess)
+		if d.aws.Credentials == nil && d.ec2MetadataAvailable(sess) {
+			creds := ec2rolecreds.NewCredentials(sess)
+			ec2s = ec2.New(sess, &aws.Config{Credentials: creds})
+		} else {
+			ec2s = ec2.New(sess)
+		}
 	}
 	tg = &config.TargetGroup{
 		Source: *d.aws.Region,