feat(detectors): add EC2 resource detection via IMDSv2

[karthikeyangs9]

Summary

• Add native EC2 instance metadata and tag detection to the go-agent using IMDSv2
• New LAST9_CLOUD_RESOURCE_DETECTION=aws env var to opt-in (zero cost when unset)
• Custom resource.Detector in detectors/aws/ — no AWS SDK dependency

Motivation

The go-agent's createResource() has no cloud resource detection. Users running on EC2 miss valuable attributes (cloud.region, host.id, instance tags) that are critical for filtering and grouping telemetry in Last9.

Previously, the only workaround was a shell script enriching OTEL_RESOURCE_ATTRIBUTES before the binary starts. This PR makes it native to the agent.

What it does

When LAST9_CLOUD_RESOURCE_DETECTION=aws is set, the agent's createResource() adds an EC2 detector that:

1. Fetches IMDSv2 token (PUT /latest/api/token with TTL header)
2. Reads instance metadata: instance-id, availability-zone, instance-type, hostname
3. Derives region from AZ (e.g., us-east-1a → us-east-1)
4. Fetches instance tags from /latest/meta-data/tags/instance/ (requires "Allow tags in instance metadata" enabled on the EC2 instance)

Resource attributes produced

Attribute	Source
cloud.provider=aws	Always (when detector runs)
cloud.region	Derived from AZ
cloud.availability_zone	IMDS
host.id	Instance ID
host.type	Instance type
host.name	Hostname
ec2.tag.<Key>	IMDS tags endpoint

Graceful degradation

• Not on EC2? IMDS is unreachable → detector returns empty resource, zero attributes, no error
• Tags not enabled? /latest/meta-data/tags/instance returns 404 → tags skipped, metadata still collected
• Partial IMDS failure? Each metadata field is fetched independently; missing fields are simply omitted
• 2-second timeout on all IMDS calls — never blocks startup

Files changed

File	Change
config/config.go	Added CloudResourceDetection field, reads LAST9_CLOUD_RESOURCE_DETECTION
agent.go	Imports detector, adds resource.WithDetectors() when config is "aws"
detectors/aws/ec2.go	New — EC2 resource detector (IMDSv2 metadata + tags)
detectors/aws/ec2_test.go	New — 5 unit tests with mock IMDS server

Usage

export LAST9_CLOUD_RESOURCE_DETECTION=aws

No code changes required — agent.Start() picks it up automatically.

Test plan

• TestDetect_FullMetadataAndTags — all metadata + 3 tags returned correctly
• TestDetect_NoTags — tags endpoint missing, core metadata still works
• TestDetect_IMDSUnreachable — non-EC2 env returns empty resource, no error
• TestDetect_PartialMetadata — only instance-id available, no region/AZ
• TestDetect_TokenValidation — IMDSv2 token sent on all metadata requests
• All existing tests pass (go test ./... -race)
• go vet ./... clean