Update the README to mention that a C API is also available.

The Metrics Library: libmetrics

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libmetrics is a small library that implements the basic C++ API for

metrics collection. All metrics collection is funneled through this

library. The easiest and recommended way for a client-side module to

collect user metrics is to link libmetrics and use its APIs to send

libmetrics is a small library that implements the basic C and C++ API

for metrics collection. All metrics collection is funneled through

this library. The easiest and recommended way for a client-side module

to collect user metrics is to link libmetrics and use its APIs to send

metrics to Chrome for transport to UMA. In order to use the library in

a module, you need to do the following:

  complete API documentation in metrics_library.h under

  src/platform/metrics/.

  For more information on the C API see c_metrics_library.h.

- On the target platform, shortly after the sample is sent it should

  be visible in Chrome through "about:histograms".

Logging.CrashCounter

Network.TimeToDrop

Platform.BootTime

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