add observability prelude module

The reason to do a wildcard import of the prelude instead of something
like

    // src/observability/prelude.rs
    pub(crate) use tracing::*;

    // elsewhere
    use crate::observability::prelude as o;
    o::warn!("something");

is that we can't import traits like tracing::Instrument that way.

I'm generally not a fan of wildcard imports, but I think it can be okay
when it's a module in the same crate. There aren't really an backwards
compatibility hazards because it's your own code.

src/observability/prelude.rs

@@ -0,0 +1,14 @@
+//! Common tracing-related items intended to be wildcard imported in most
+//! modules:
+//!
+//! ```
+//! use crate::observability::prelude::*;
+//! ```
+//!
+//! This avoids diff churn in the imports when adding/removing log statements.
+//!
+//! [`tracing`] is re-exported as `t`, to avoid name collisions.
+#![allow(unused)]
+
+pub(crate) use tracing as t;
+pub(crate) use tracing::Instrument;