Optimize JSON metric extraction by reusing parsed data

[balookrd]

Motivation

• Large raw JSON payloads (e.g. ~50MB) were repeatedly unmarshalled and had JSONPath expressions reparsed for every metric/label extraction, increasing CPU and GC pressure and slowing scrapes.

Description

• Parse the raw payload once per scrape by calling json.Unmarshal in JSONMetricCollector.Collect and reuse the resulting interface{} for all extractions.
• Add precompiled JSONPath parsers to JSONMetric (keyParser, valueParser, labelsParsers, timestampParser) and a compileJSONPaths helper to build them.
• Change extractValue, extractLabels, and timestampMetric to operate on parsed interface{} data and use precompiled *jsonpath.JSONPath parsers, and avoid marshal/unmarshal round-trips for ObjectScrape items.
• Compile JSONPath expressions when creating metrics in CreateMetricsList so parsing errors surface early and runtime can reuse parsers; modified files are exporter/collector.go and exporter/util.go.