feat: add prometheus metrics endpoint

[glefer]

This pull request introduces a Prometheus metrics endpoint to the Docker Volume Analyzer application. The /metrics endpoint provides detailed metrics about Docker volumes, enabling integration with Prometheus for monitoring and observability.

Key Changes

Added a /metrics route to the Flask application.
Exposed the following Prometheus metrics:

• docker_volumes_total: Total number of Docker volumes.
• docker_volume_size_bytes: Size of individual Docker volumes in bytes, labeled by volume name.

Output example

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
src/docker_volume_analyzer
__init__.py	0	0	100%
docker_client.py	82	0	100%
errors.py	3	0	100%
filesystem.py	51	0	100%
main.py	9	0	100%
tui.py	163	0	100%
volume_manager.py	33	0	100%
web.py	21	0	100%
TOTAL	362	0	100%