add quick logic for jobs to be passed in the uri path

[mplachter]

Quick implementation for passed job name in URI to be added to metrics as labels, this is to prevent different metrics from clobbering together from different jobs.

Before Benchmark

pkg: prom-aggregation-gateway
BenchmarkConcurrentAggregate/metric_type_simpleGauge-10                   160633             76634 ns/op           87347 B/op       1313 allocs/op
BenchmarkConcurrentAggregate/metric_type_fullMetrics-10                    85020            141702 ns/op          194174 B/op       3773 allocs/op
BenchmarkConcurrentAggregate/metric_type_multiLabel-10                    228883             51287 ns/op           64626 B/op        673 allocs/op
BenchmarkConcurrentAggregate/metric_type_multiLabelIgnore-10              236571             50984 ns/op           65026 B/op        703 allocs/op
BenchmarkConcurrentAggregate/metric_type_labelFields-10                   229822             49480 ns/op           61830 B/op        553 allocs/op
BenchmarkConcurrentAggregate/metric_type_reorderedLabels-10               247396             48567 ns/op           63026 B/op        603 allocs/op
BenchmarkConcurrentAggregate/metric_type_ignoredLabels-10                 247699             48915 ns/op           64866 B/op        703 allocs/op
PASS
ok      prom-aggregation-gateway        88.855s

After Benchmark

pkg: prom-aggregation-gateway
BenchmarkConcurrentAggregate/metric_type_simpleGauge-10                   151737             80920 ns/op           90707 B/op       1403 allocs/op
BenchmarkConcurrentAggregate/metric_type_fullMetrics-10                    83394            149929 ns/op          197534 B/op       3893 allocs/op
BenchmarkConcurrentAggregate/metric_type_multiLabel-10                    212082             54311 ns/op           65425 B/op        693 allocs/op
BenchmarkConcurrentAggregate/metric_type_multiLabelIgnore-10              228790             51045 ns/op           66146 B/op        733 allocs/op
BenchmarkConcurrentAggregate/metric_type_labelFields-10                   251012             47880 ns/op           62790 B/op        583 allocs/op
BenchmarkConcurrentAggregate/metric_type_reorderedLabels-10               257607             50677 ns/op           64146 B/op        633 allocs/op
BenchmarkConcurrentAggregate/metric_type_ignoredLabels-10                 228663             49682 ns/op           65986 B/op        733 allocs/op
PASS
ok      prom-aggregation-gateway        89.613s

This was quick and dirty, this can be optimized.