Add disk usage metrics to hostmetrics dashboard

[oleksandr-shostak]

PR Description

## Summary
This PR adds comprehensive disk usage monitoring capabilities to the hostmetrics dashboard, including disk usage tables and percentage graphs per mountpoint.

## Changes Made

### Added Features
1. **Disk Usage Table** (Overview section)
   - Shows disk usage per mountpoint with used and total space in bytes
   - Displays Used % and Free % columns calculated from used/total space
   - Grouped by mountpoint and device

2. **Disk Usage Percentage Graph** (Disk section)
   - Time-series graph showing disk usage percentage over time
   - One line per mountpoint showing usage percentage (0-100%)
   - Helps track disk space trends and identify which disks are filling up

3. **Disk Usage Percentage Table** (Disk section)
   - Table showing current disk usage percentage per mountpoint
   - Quick reference for which disks are near capacity

### Technical Details
- Uses `system.filesystem.usage` metric from hostmetrics receiver
- Calculates percentages using formula: `(used / total) * 100` for used percentage
- Calculates free percentage using formula: `((total - used) / total) * 100`
- All widgets properly organized under the Disk section
- Units configured as `bytes` (auto-formats to KB/MB/GB/TB) and `percentunit` for percentages

### Files Changed
- `hostmetrics/hostmetrics.json` - Added 3 new widgets (table + 2 graphs) with proper layout and queries

## Testing
- ✅ JSON syntax validated
- ✅ All widgets properly configured with queries
- ✅ Layout positions adjusted correctly
- ✅ Widgets organized under Disk section