I had the same problem. It’s not telemetry, it’s old cached files. Delete “_build” and “deps” folders and run “mix deps.get” and “mix deps.compile”. Everything ok now.

I’m seeing this (occasional :telemetry compile fails) too. I’m not sure how to repro it as it seems to happen randomly once and awhile. I’ve seen it on my local OSX machine as well as on a circleci/elixir:1.9.1-node container.

The relevent part from my CircleCI config looks like this:

elixir_tests:
    docker:
      - image: circleci/elixir:1.9.1-node
    working_directory: ~/repo/api/my_app
    steps:
      - checkout:
          path: ~/repo

      - restore_cache:
          keys:
            - ex-deps-{{ checksum "mix.exs" }}-{{ checksum "mix.lock" }}
            - ex-deps-

      - run:
          name: Install hex and rebar3
          command: mix local.hex --force && mix local.rebar --force

      - run:
          name: App Dependencies
          command: MIX_ENV=test mix do deps.get --only test, deps.compile, compile

      - save_cache:
          paths:
            - deps
          key: ex-deps-{{ checksum "mix.exs" }}-{{ checksum "mix.lock" }}

      - run:
          name: ExUnit Tests
          command: mix test --cover

      - store_artifacts:
          path: cover
          destination: cover

mix deps.clean telemetry fixes it up if I SSH in and try to interactively manipulate the build.

That might actually be an issue, mix local.rebar installs versions it is known to work with, you should not use a different rebar when compiling mix projects.