Split metrics (cached) and tracing (uncached) label decoding

[bobrik]

In tracing there is no concern with cardinality and one can have PID as a label. With caching enabled, this means that we grow the cache map indefinitely, leaking memory until OOM stops us. Let's disable caching for tracing instead.

[bobrik]

Perhaps an LRU with 1k entries per config is a better option here.

[bobrik]

With LRU there's overhead in terms of CPU for the metrics case:

ivan@vm:~/projects/ebpf_exporter$ ~/go/bin/benchcmp /tmp/old.txt /tmp/new.txt
benchcmp is deprecated in favor of benchstat: https://pkg.go.dev/golang.org/x/perf/cmd/benchstat
benchmark                   old ns/op     new ns/op     delta
BenchmarkCache/direct-8     382           395           +3.30%
BenchmarkCache/cached-8     20.9          35.7          +70.30%

benchmark                   old allocs     new allocs     delta
BenchmarkCache/direct-8     13             13             +0.00%
BenchmarkCache/cached-8     0              0              +0.00%

benchmark                   old bytes     new bytes     delta
BenchmarkCache/direct-8     216           216           +0.00%
BenchmarkCache/cached-8     0             0             +0.00%

There is no overhead for metrics with separate methods.

For tracing we have a set of nearly unique labels that are not worth caching:

        - name: trace_id
          size: 16
          decoders:
            - name: hex
        - name: parent_span_id
          size: 8
          decoders:
            - name: hex
        - name: span_id
          size: 8
          decoders:
            - name: hex
        - name: span_monotonic_timestamp_ns
          size: 8
          decoders:
            - name: uint
        - name: span_duration_ns
          size: 8
          decoders:
            - name: uint

Given that we cache the whole input with unique and non-unique fields, there's zero chance of actually caching anything for tracing. There doesn't seem to be any point in LRU then.