Serve config at HTTP endpoint

[annanay25]

Signed-off-by: Annanay annanayagarwal@gmail.com

What this PR does:
Adds a /config handler to serve the config that tempo is running with. Makes it easier to debug in non-k8s environments.

Long term, we should add an API module similar to Cortex.

Which issue(s) this PR fixes:
Fixes #nada

Checklist

• NA Tests updated
• NA Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[joe-elliott]

Tempo can include sensitive information in its config. We either need to find a way to redact that or perhaps only serve this endpoint if it's configured.

[annanay25]

Good point, I had not considered that. I was following suit with Cortex, which serves config by default I guess.

[annanay25]

Okay, cortex is clever :) - https://github.com/cortexproject/cortex/blob/master/pkg/util/flagext/secret.go

I'll reuse this.

[annanay25]

Results in

storage:
    trace:
        backend: s3
        local:
            path: ""
        gcs:
            bucket_name: ""
            chunk_buffer_size: 10485760
            endpoint: ""
            insecure: false
        s3:
            bucket: tempo
            endpoint: minio:9000
            access_key: '********'
            secret_key: '********'
            insecure: true
            part_size: 0
            signature_v2: false