Changes

This PR introduces semantic conventions for DHCP (Dynamic Host Configuration Protocol) instrumentation, covering both DHCPv4 (RFC 2131/RFC 2132) and DHCPv6 (RFC 8415).

Attribute Registry:

• Common DHCP attributes (message type, operation name, request ID, status codes, client/server identifiers, lease duration)
• DHCPv4 protocol fields and options (ciaddr, yiaddr, giaddr, chaddr, subnet mask, DNS servers, etc.)
• DHCPv4 relay agent information (Option 82)
• DHCPv6 protocol fields and options (IA address, preference, status codes, DNS servers, etc.)
• DHCPv6 DUID attributes

Span Definitions:

• Client spans for DHCP message transmission
• Server spans for DHCP message processing
• Span events for message sent/received, address allocation/release, lease expiration

Metric Definitions:

• Client metrics: request duration, lease duration
• Server metrics: request duration, pool usage/limit/utilization, active leases
• Relay metrics: request duration

Documentation:

• README with overview and links to RFCs
• Span conventions documentation
• Metric conventions documentation

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
• Links to the prototypes or existing instrumentations (when adding or changing conventions)