feat: add span exclusion sampler and fix context handling

[Taure]

Summary

• Fix context token handling: replace otel_ctx:clear() with otel_ctx:detach(PrevCtxToken) in terminate/3 to properly restore previous OTel context instead of destroying it
• Add otel_nova_sampler module implementing otel_sampler behaviour for excluding server spans by name (e.g. health checks, metrics endpoints)
• Add opentelemetry as a main dependency (required for otel_sampler behaviour)

Sampler usage

{opentelemetry, [
    {sampler, {parent_based, #{
        root => {otel_nova_sampler, [<<"GET /metrics">>, <<"GET /healthz">>]}
    }}}
]}.

Test plan

• New test: stream_handler_restores_context_on_terminate — verifies context is restored, not cleared
• New suite: otel_nova_sampler_SUITE — 3 tests (excludes matching server spans, records non-matching, records non-server spans even if name matches)
• All 33 tests pass
• rebar3 fmt --check — clean
• rebar3 xref — 0 warnings
• rebar3 dialyzer — 0 warnings