fix(ddtrace/tracer): align OTLP trace-metrics export with span metrics connector conventions

[mabdinur]

What does this PR do?

Fixes several OTLP trace-metrics compliance gaps in stats_to_otlp_metrics.go, found during a cross-tracer audit against the OTel Span Metrics Connector's expected attribute conventions:

• process_tags: emit a single datadog.process_tags resource attribute (arrayValue) instead of per-key datadog.<key> resource attributes.
• is_trace_root: emit datadog.is_trace_root as a data-point attribute, reading the previously-unused ClientGroupedStats.IsTraceRoot field.
• peer_tags: emit datadog.peer_tags as a data-point attribute (arrayValue), mirroring the same peer-tag-key allowlist the /v0.6/stats path already uses, instead of always suppressing it.
• additional_metric_tags: emit each configured tag as its own unprefixed data-point attribute (support for this is still evolving across most SDKs).
• span.kind: canonicalize to the Span Metrics Connector's SPAN_KIND_* string convention (e.g. SPAN_KIND_SERVER) instead of the raw lowercase Datadog value.
• status.code: emit the Span Metrics Connector's STATUS_CODE_OK/STATUS_CODE_ERROR string convention (previously an OTel int enum), unconditionally on every data point.

All of the above except additional_metric_tags are gated by !otelMode, consistent with the existing datadog.* attribute suppression in OTel-semantics mode. http.status_code/gRPC status handling and service.name (already correct) are unaffected.

Motivation

Companion cross-tracer fix: DataDog/system-tests#7363

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• New code is free of linting errors. You can check this by running make lint locally.
• New code doesn't break existing tests. You can check this by running make test locally.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• All generated files are up to date. You can check this by running make generate locally.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild. Make sure all nested modules are up to date by running make fix-modules locally.

[mabdinur]

Closing: opened against the wrong local branch due to a working-directory mix-up (not munir/otlp-trace-metrics-fixes). Reopening correctly.