feat: add metrique-battery-pack

[jlizen]

New crate: metrique-battery-pack

Ref: battery-pack repo, authoring guide

Motivation

metrique has 13 crates in its workspace. A basic "define a struct, set up a sink, emit on drop" workflow touches three crates and four or five trait imports from different modules. Good candidate for a battery pack.

Most of the workspace crates are already re-exported by metrique behind feature flags. The battery pack should enable those flags rather than adding the underlying crates as separate dependencies.

Default set

Just metrique itself.

Features

Feature	What it enables	Notes
emf	metrique/emf	CloudWatch Embedded Metric Format
json	metrique/json	Plain JSON output
aggregation	metrique-aggregation	#[aggregate], Histogram, Sum, KeyedAggregator, WorkerSink
util	metrique-util	State<T> for shared runtime state

Templates

Multiple templates, each with tests demonstrating test-util feature enablement. A shared METRIQUE.md (trait import cheat sheet, next steps, how to switch formats) is included in all templates via [[files]]. Templates can use MiniJinja placeholder variables (e.g., format = emf/json) to branch on sink setup, Cargo features, and imports.

cargo bp new metrique --name my-service
# prompts for template selection

Template	What it scaffolds
default	Async runtime, sink initialization (MiniJinja format placeholder for emf/json), global metrics setup (merge_globals), sample #[metrics] request handler with emit on drop
library	#[metrics(subfield)] function using Instrumented, returning metrics to the caller
aggregation	#[aggregate] with Histogram/Sum, embedded via Aggregate<T>, KeyedAggregator