feat: experimental tracing channels support

[logaretm]

Adds Node.js tracing channels for all storage operations, enabling observability and performance monitoring.

Channel Naming Convention

All channels use the unstorage.${operation} convention which when coupled with tracing channels produce the following diagnostic channels:

• tracing:unstorage.${operation}:start
• tracing:unstorage.${operation}:end
• tracing:unstorage.${operation}:asyncStart
• tracing:unstorage.${operation}:asyncEnd
• tracing:unstorage.${operation}:error

Which matches nicely with fastify but there isn't a lot of convention for them other than the Node.js recommendation.

Traced operations

• Item operations: hasItem, getItem, setItem, setItems, removeItem
• Raw operations: getItemRaw, setItemRaw
• Meta operations: getMeta (uses getItem channel with meta: true)
• Batch operations: getItems, setItems
• Utility operations: getKeys, clear

Tracing context includes

• keys[] - keys involved in the operation
• base - mount point handling the operation
• driver - driver name and options (single-key operations only)
• meta - flag for metadata operations

We can probably add more, in the Nuxt unstorage instrumentation we only use those.

Example

import { tracingChannel } from 'node:diagnostics_channel';

const channel = tracingChannel('unstorage.getItem');

channel.subscribe({
  start: (data) => {
    // start timing, or spans or whatever...
  },
  asyncEnd: (data) => {
    console.log(`${data.driver?.name} fetched ${data.keys[0]} in ${duration}ms`);
  },
  error: (data) => {
    console.error(`Error in ${data.driver?.name}:`, data.error);
  }
});

All operations emit standard tracing events: start, end, asyncStart, asyncEnd, and error.

---

⚠️ Current Blocker: unstorage can be used in browser, so we need to shim it in this case, or perhaps provide an alternative.

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[logaretm]

@pi0 I refactored the logic here to use process.getBuiltIn..., separated the tracing into its own subpath export, and reduced the logic size by generalizing a trace wrapper.

[codecov]

Codecov Report

❌ Patch coverage is 95.55556% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.60%. Comparing base (a52bf75) to head (50dded0).

Files with missing lines	Patch %	Lines
src/tracing.ts	95.55%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #707      +/-   ##
==========================================
+ Coverage   91.77%   92.60%   +0.83%     
==========================================
  Files           5        6       +1     
  Lines         401      446      +45     
  Branches      126      143      +17     
==========================================
+ Hits          368      413      +45     
  Misses         33       33              

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