Change description

This PR allows users of the library to define additional_tags that should be included in HTTP metrics from the Phoenix plugin.

What problem does this solve?

Folks may wish to tag HTTP events with information other than the default tags (status, route, etc.). For example, someone might want to tag events with authenticated="true" based on the authentication status of the request. While this kind of custom tagging can be abused, when used well, it can greatly improve one's ability to query the collected data.

Example usage

First, configure the Phoenix plugin to look for additional tags:

def plugins do
  [
    {PromEx.Plugins.Phoenix,
     router: MyApp.Router,
     endpoint: MyApp.Endpoint,
     additional_tags: [:authenticated]}
  ]
end

Then, set data in the connection struct's private data using a matching prefixed key:

# Authentication Plug
def call(conn, opts) do
  # ...
  Plug.Conn.put_private(conn, :prom_ex_authenticated, true)
end

Additional details and screenshots

I initially implemented this PR in a way that required additional tags to be stored in a nested :prom_ex map in the private connection data (you can see the first commit for that version). This made it difficult to store metadata, however, because callers would have to check for the existence of the :prom_ex map and either create it or merge data into it as appropriate. Using top-level interpolated keys solves this issue while still preventing naming collisions with other private data.

Checklist

• I have added unit tests to cover my changes.
• I have added documentation to cover my changes.
• My changes have passed unit tests and have been tested E2E in an example project.