Replace OpenCensus with OpenTelemetry

[matt0x6F]

What is the problem I am trying to address?

#1771: replace OpenCensus with OpenTelemetry. OpenCensus is deprecated and was
sunset in 2023.

#1891 was a draft that took the OpenCensus bridge route — keep the OpenCensus API and
route its spans into an OTel provider. The bridge works but it leaves the deprecated
dependency in the tree and only defers the real migration. Since the instrumentation all
goes through pkg/observ rather than calling OpenCensus directly, going native isn't much
more work than the bridge, so this does that instead and drops OpenCensus entirely.

How is the fix applied?

pkg/observ is a facade — call sites use observ.StartSpan and observ.Record*, not the
OpenCensus API. 45 of the 46 files that start spans only call .End() on what they get
back, so changing StartSpan to return an OTel span leaves them untouched. The migration
is mostly contained to the facade.

• Traces export over OTLP (gRPC). RegisterExporter builds an otlptracegrpc exporter and
  an SDK TracerProvider, sets it global, and returns a Shutdown for cleanup.
  TraceExporter is now a toggle: otlp or empty. TraceExporterURL is the collector
  endpoint, and when it's empty the standard OTEL_EXPORTER_OTLP_* env vars are honored.
  The default moves from :14268 (Jaeger) to :4317 (OTLP).
• Metrics go through an OTel MeterProvider backed by a Prometheus reader on the same
  /metrics route. The three custom instruments (cache_lookup_total,
  upstream_fetch_total, upstream_fetch_duration_seconds) keep their names, including the
  proxy_ prefix, so existing scrape configs and dashboards keep working. HTTP server and
  client metrics now come from otelhttp, which means those metric names change to the OTel
  semconv names.
• ochttp.Handler/ochttp.Transport become otelhttp.NewHandler/otelhttp.NewTransport.
• stasher.go was the one file reaching under the facade (trace.NewContext); it now uses
  trace.ContextWithSpan.
• The jaeger, datadog, and stackdriver exporters are gone. Asking for one logs a
  message telling you to use OTLP and carries on — like an unset or unknown exporter, it
  doesn't stop Athens from booting, you just don't get traces.
• The OpenTelemetry SDK (v1.43.0) was already in the tree as an indirect dependency, so this
  mostly promotes it to direct and drops go.opencensus.io and the three
  contrib.go.opencensus.io/exporter/* modules.

Sampling is configurable. development still samples every trace. Outside it, the new
ATHENS_TRACE_SAMPLING_FRACTION (TraceSamplingFraction in the config file) sets the
fraction of root traces sampled — 0.0 to 1.0, defaulting to 1.0 — via
ParentBased(TraceIDRatioBased(...)), so child spans follow their parent's decision.

What GitHub issue(s) does this PR fix or close?

Fixes #1771, #2026

Note: this drops direct datadog and stackdriver export. Athens still exposes metrics
on /metrics for Prometheus, and traces can reach either through an OTLP collector. Anyone
on ATHENS_TRACE_EXPORTER=datadog/stackdriver needs to point at a collector with
ATHENS_TRACE_EXPORTER=otlp instead — an unsupported value isn't fatal, Athens logs a
message and boots without tracing. The three custom metric names are unchanged; the HTTP
metric names change to the OTel conventions.