NUTCH-3134 Add latency metrics with percentile support to Fetcher, Parser, and Indexer

[lewismc]

PR for NUTCH-3134. Notably, this PR introduces a new Class named LatencyTracker.java which tracks latency metrics. The implementation wraps the TDigest data structure to collect latency samples and emit Hadoop counters with count, sum, and percentile values (p50, p95, p99). Note this is limited to Fetcher, Parser and Indexer jobs right now but could certainly be extended to other jobs in the future.

One note for any reviewers, please sanity check

1. latency start ands stop boundaries are accurate.
2. counters are emitted at the correct times.

Thanks for any review. Local testing is favorable. My next step will be to share my WIP Nutch observability solution via user@ .

[sebastian-nagel]

Looks good! And works.

Counters from a small local test crawl (content served from a local Apache httpd):

fetch_latency_count_total=27
fetch_latency_p50_ms=13
fetch_latency_p95_ms=28
fetch_latency_p99_ms=61
fetch_latency_sum_ms=421
``