Add metric debuginfo_upload_request_bytes

[secfree]

Currently, parca-agent exposes the following network-related metrics:

• process_network_receive_bytes_total
• process_network_transmit_bytes_total
• sample_write_request_bytes
• stacktrace_write_request_bytes

The process_network_* metrics are tied to the process's network namespace. When parca-agent runs outside a container or dedicated network namespace, these metrics capture the entire node's network traffic rather than parca-agent-specific traffic.

This PR adds a new metric debuginfo_upload_request_bytes to track the total bytes uploaded in debuginfo upload requests. This complements the existing sample_write_request_bytes and stacktrace_write_request_bytes metrics, providing complete visibility into parca-agent's application-level network usage.

[brancz]

Thank you!