fix(metrics/otel): bracket IPv6 host literals in exporter endpoint URL

[1fanwang]

Problem

get_otel_data_exporter() builds the OTLP exporter endpoint URL with a raw f-string:

endpoint_str = f"{protocol}://{host}:{port}/v1/{endpoint_suffix}"

When host is an IPv6 literal (e.g. ::1, 2001:db8::1), the resulting URL is invalid per RFC 3986 §3.2.2 — the v6 host has to be enclosed in [...] so the : separators don't conflict with the host:port delimiter:

http://::1:4318/v1/metrics              # invalid
http://2001:db8::1:4318/v1/metrics      # invalid

The OTLP exporter then either fails to parse the URL or interprets the trailing port digits as part of the v6 address.

Fix

Add a small _format_url_host() helper in shared/observability/.../common.py that brackets bare IPv6 literals and leaves hostnames, IPv4 literals, and already-bracketed v6 strings unchanged. The helper accepts str | None so the existing error-logging path (where the deprecated airflow host/port config can resolve to None) keeps its shape.

Tests

Extended test_config_priorities with four new parametrised cases:

• ::1 → http://[::1]:4318/v1/metrics
• 2001:db8::1 → http://[2001:db8::1]:4318/v1/metrics
• [::1] (already bracketed) → preserved
• 10.0.0.1 (v4) → passes through unchanged

Closes #66811