fix(metrics): improve error messages and fix timestamp format

[platinummonkey]

Summary

Fixes #29 - Improves error handling in metrics commands to provide detailed API error messages and fixes timestamp format bug causing "Queries before 2010 are invalid" errors.

Changes

Error Message Improvements

• Extract and display full API response body in error messages
• Add request context (query, timestamps, filters, tags) to errors
• Include troubleshooting tips for common issues
• Applied to all metrics commands: query, list, metadata get/update, and submit

Timestamp Format Fix

• Root Cause: API v2 timeseries endpoint expects timestamps in milliseconds, not seconds
• Before: Using from.Unix() (seconds) → e.g., 1770772813
• After: Using from.UTC().UnixMilli() (milliseconds) → e.g., 1770772813000
• Ensures timestamps are converted to UTC before sending to API

Example

Before (generic error):

Error: failed to query metrics: 400 Bad Request (status: 400)

After (detailed error):

Error: failed to query metrics: 400 Bad Request
Status: 400
API Response: {"errors":["Queries before 2010 are invalid"]}

Request Details:
- Query: avg:system.cpu.user{*}
- From: 2026-02-10T19:20:13Z (Unix: 1770772813)
- To: 2026-02-10T20:20:13Z (Unix: 1770776413)

Troubleshooting:
- Verify your query syntax is correct (e.g., avg:metric.name{filter})
- Check that the time range is valid
- Ensure the metric exists and has data in the specified time range
- Confirm you have proper permissions to access the metric

Testing

• All existing tests pass
• Verified build completes successfully
• Manually tested error output format

Related Issues

Closes #29

---

🤖 Generated with Claude Code

[github-actions]

📊 Test Coverage Report

Overall Coverage: 80.4%

Threshold: 80% ✅

Coverage by Package

## Coverage by Package

- github.com/DataDog/pup/pkg/auth/callback/server.go:40: 81.2%
- github.com/DataDog/pup/pkg/auth/dcr/client.go:28: 100.0%
- github.com/DataDog/pup/pkg/auth/dcr/types.go:24: 100.0%
- github.com/DataDog/pup/pkg/auth/oauth/client.go:22: 100.0%
- github.com/DataDog/pup/pkg/auth/oauth/pkce.go:24: 85.7%
- github.com/DataDog/pup/pkg/auth/storage/factory.go:53: 94.7%
- github.com/DataDog/pup/pkg/auth/storage/keychain.go:44: 42.9%
- github.com/DataDog/pup/pkg/auth/storage/storage.go:58: 71.4%
- github.com/DataDog/pup/pkg/auth/types/types.go:23: 100.0%
- github.com/DataDog/pup/pkg/client/client.go:32: 94.4%
- github.com/DataDog/pup/pkg/config/config.go:22: 100.0%
- github.com/DataDog/pup/pkg/formatter/formatter.go:31: 100.0%
- github.com/DataDog/pup/pkg/useragent/useragent.go:32: 100.0%
- github.com/DataDog/pup/pkg/util/time.go:20: 95.8%

## Summary

total:								(statements)		80.4%

---

📈 Coverage Status: ✅ PASSED - Coverage meets minimum threshold

_{Updated for commit 0a3adb2}