add x-client-id to markdown documentation examples for /query/[metrics|batch]

basically need to find a neat way of adding "x-client-id: my-app" to the example cURLs given in https://spotify.github.io/heroic/docs/api/post-query-metrics and https://spotify.github.io/heroic/docs/api/post-query-batch .

it's not that easy since we want this only to be added to these two endpoints. Probably need to create a "sub class" of whatever spits out the curl -XPOST ... text.