feat: add mainmatter telemetry workshop example

tracing/mainmatter-workshop/README.md

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+# Telemetry Workshop Solution
+
+## Overview
+
+A solution to the capstone project at the end of [Mainmatter's telemetry workshop](https://github.com/mainmatter/rust-telemetry-workshop).
+
+As stated in the exercise brief, this example will:
+
+- Configure a `tracing` subscriber that exports data to both Honeycomb and stdout, in JSON format;
+- Configure a suitable panic hook;
+- Configure a `metric` recorder that exposes metric data at `/metrics`~~, using a different port than your API endpoints~~ (this example shows how to use the existing HTTP server);
+- Add one or more middleware that:
+  - Create a top-level INFO span for each incoming request;
+  - Track the number of concurrent requests using a gauge;
+  - Track request duration using a histogram;
+  - Track the number of handled requests All metrics should include success/failure as a label.
+
+## Usage
+
+```console
+$ cd tracing/mainmatter-workshop
+$ cargo run
+```