feat(telemetry): support gRPC transport for OTLP exporters

[aribault]

Motivation

StrandsTelemetry.setup_otlp_exporter and the OTLP-metrics path inside setup_meter hard-code the HTTP variant of OTLPSpanExporter / OTLPMetricExporter. Users running their telemetry through the OpenTelemetry Collector (or vendor agents that prefer gRPC) currently have to bypass the helpers entirely and reconstruct the providers themselves.

This change adds first-class gRPC support while keeping HTTP as the default, matching Unshure's direction in #689.

Resolves: #689

Note: This PR is stacked on #2324 (issue #843). Until that lands on main, the GitHub diff here will include #2324's setup_meter provider_kwargs commit. Once #2324 merges, this branch will rebase onto fresh main and the diff will collapse to just the gRPC-related changes.

Public API Changes

setup_otlp_exporter now accepts a protocol parameter, and setup_meter accepts otlp_protocol. Resolution order is explicit arg → OTEL_EXPORTER_OTLP_PROTOCOL env var → http/protobuf (matching the OTel SDK convention). When \"grpc\" is selected and the optional dependency is missing, an ImportError surfaces a pip install hint pointing at the new otel-grpc extra.

# Before — no way to choose gRPC short of bypassing setup_otlp_exporter entirely
StrandsTelemetry().setup_otlp_exporter(endpoint=\"https://collector:4318/v1/traces\")

# After — explicit protocol selection
StrandsTelemetry().setup_otlp_exporter(
    protocol=\"grpc\",
    endpoint=\"https://collector:4317\",
)

# Or via the standard OTel env var
# OTEL_EXPORTER_OTLP_PROTOCOL=grpc
StrandsTelemetry().setup_otlp_exporter()  # picks up env

# Same shape on the metrics side
StrandsTelemetry().setup_meter(enable_otlp_exporter=True, otlp_protocol=\"grpc\")

New Optional Dependency

A new otel-grpc extra in pyproject.toml:

otel-grpc = [\"opentelemetry-exporter-otlp-proto-grpc>=1.30.0,<2.0.0\"]

The existing otel extra is untouched (HTTP-only). Users opt into gRPC explicitly via pip install 'strands-agents[otel-grpc]'. The all meta-extra now includes otel-grpc so users on [all] get both transports.

Existing callers (no protocol argument, otel extra installed) keep their previous behavior with no install-size growth.