Use reflection rather than MethodHandles

[felixbarny]

This is a workaround for a bug in the C2 compiler in Java 7 which leads to a segfault
closes #458

[felixbarny]

Snapshot for these changes: elastic-apm-agent-1.3.1-SNAPSHOT.jar.zip

[codecov-io]

Codecov Report

Merging #487 into master will not change coverage.
The diff coverage is 75%.

@@            Coverage Diff            @@
##             master     #487   +/-   ##
=========================================
  Coverage     64.32%   64.32%           
  Complexity       68       68           
=========================================
  Files           164      164           
  Lines          6197     6197           
  Branches        716      716           
=========================================
  Hits           3986     3986           
  Misses         1965     1965           
  Partials        246      246

Impacted Files	Coverage Δ	Complexity Δ
...astic/apm/agent/metrics/builtin/SystemMetrics.java	71.05% <75%> (ø)	0 <0> (ø)	⬇️

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