feat: make Prometheus remote-write endpoint optional

[cbermudez97]

Summary

The full benchmark workflow does not require a Prometheus remote-write endpoint — k6 already writes a summary.json, and the runner has a complete path for building per-client/per-method metrics from it. Until now the CLI forced Prometheus by defaulting --prometheus to http://localhost:9090, rejecting an empty value, and constructing PrometheusRW unconditionally.

This PR makes Prometheus optional and disabled by default.

Behavior

--prometheus value	k6 remote-write	Client metrics source
empty (new default)	not enabled	summary.json
a URL (opt-in)	enabled, as before	Prometheus query, summary.json fills gaps

Changes

• runner/cmd/benchmark.go — --prometheus defaults to empty; removed the --prometheus is required guard; PrometheusRW is only built when an endpoint is provided (cfg.Outputs stays non-nil).
• runner/metrics/results_collection.go — CollectClientsMetrics routes to a new summary-only collection path when Prometheus is not configured (instead of returning "no outputs configured"), filling every client/method pair from summary.json via the existing fallback. Extracted shared newClientsMetricsSkeleton and finalizeClientMetrics helpers so both paths behave identically after data load.
• runner/generator/k6_generator.go — defensive nil-Outputs guard; k6 already skipped the experimental-prometheus-rw output when PrometheusRW was nil.
• runner/analyzer/performance_analyzer.go — fix a latent 0/0 = NaN in throughput averaging for a client with no methods. The summary-only path can now surface an empty-methods client (e.g. a node returning zero successful requests), and the NaN broke JSON export.
• README.md — documented Prometheus as optional/disabled-by-default across usage examples, the migration table, and the metrics section.
• rpc-calls/eth_getStorageAt-mainnet-latest.jsonl — dropped an invalid entry with a non-hex "0xasdf" storage slot.

Tests / verification

• Added runner/metrics/results_collection_test.go covering CollectClientsMetrics with both nil Outputs and nil PrometheusRW, asserting metrics come from summary.json and totals are finalized.
• go build, go vet, and package tests pass (the pre-existing TestP99DataFlow requires a live PostgreSQL and is unrelated).
• Ran a real benchmark with no --prometheus: no required-flag error, k6 ran without remote-write, the summary.json fallback engaged, results.json was written, and no NaN in output. Opt-in run with --prometheus http://localhost:9090 is unchanged.