attributes: permit #[instrument(follows_from = …)] (tokio-rs#2093)

This PR extends the `#[instrument]` attribute to accept an optional
`follows_from = …` argument that supplies any number of
`Span::follows_from` relationships to the generated `Span`.

## Motivation

This PR resolves tokio-rs#879.

## Solution

This PR largely follows the implementation strategy articulated by
@hawkw:
tokio-rs#879 (comment)

In that comment, @hawkw suggests taking one of two approaches:
1. each `follows_from` relationship is supplied with a distinct
   `follows_from` argument
2. the `follows_from` argument is provided once, and its value is a
   **list** of indirect causes

I take the second approach, since it is slightly more flexible: it
allows for the number of indirect causes to vary at runtime.

This addition is complemented by changes to `tracing-mock` to permit
making `follows_from` assertions for testing purposes.

tracing-attributes/src/expand.rs

@@ -129,6 +129,13 @@ fn gen_block<B: ToTokens>(
         })*
     };
 
+    let follows_from = args.follows_from.iter();
+    let follows_from = quote! {
+        #(for cause in #follows_from {
+            __tracing_attr_span.follows_from(cause);
+        })*
+    };
+
     // generate this inside a closure, so we can return early on errors.
     let span = (|| {
         // Pull out the arguments-to-be-skipped first, so we can filter results