Parse const_labels correctly in exporter

[fredr]

PR Description

Config field const_labels have different types in the remotewriterexporter than it has in the main config type. This causes an error when starting the agent:

error reading exporters configuration for \"remote_write\": 1 error(s) decoding:\n\n* 'const_labels[0]' has invalid keys: key, some"

With this example config:

server:
  log_level: info
  http_listen_port: 12345

traces:
  configs:
  - name: test
    receivers:
      zipkin:
        endpoint: 0.0.0.0:9999
    remote_write:
      - endpoint: traces.local:5555
        insecure: false
        tls_config:
            insecure_skip_verify: true
    spanmetrics:
      metrics_instance: test
      const_labels:
        some: "somevalue"
        key: "value"

Notes to the Reviewer

The previous implementation also had a bug in the mapstructure field, where both value and name where named name.

PR Checklist

• CHANGELOG updated
• Documentation added
• Tests updated

[CLAassistant]

All committers have signed the CLA.

[fredr]

Test failed with

failed to start manager: inotify_add_watch /sys/fs/cgroup/devices: no space left on device

I think that is unrelated, I can run all the tests locally

[mapno]

Hi! Thanks for addressing this issue, it's an awful bug.

It'd be great if we add const_labels to the test. Please, add an entry to the CHANGELOG :)

[fredr]

It'd be great if we add const_labels to the test.

I added a new test that sets the spanmetrics section and creates a traces collector. Would you rather that I add it to the TestTraces-test?

Please, add an entry to the CHANGELOG :)

Done!

[mapno]

I added a new test that sets the spanmetrics section and creates a traces collector.

Nice! Looks good to me. Thanks!

Would you rather that I add it to the TestTraces-test?

That's fine by me. I think ideally the test should be in config_test.go, but it's fine where it is as long as it's tested. Up to you!

Thanks for the PR and addressing the comments! :)