Add a metrics example

Author: duncanpo

examples/metrics/README.md

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+# Metrics Example
+This example shows how to emit OpenTelemetry metrics from MATLAB. It uses all 3 synchronous instruments counter, updowncounter, and histogram.
+* At the beginning of the first run, initialization is necessary to create and store a global meter provider.
+* The example then enters a loop and at each iteration updates all 3 instruments. The metrics will then be exported periodically at a fixed time interval.
+
+## Running the Example
+1. Start an instance of [OpenTelemetry Collector](https://github.com/open-telemetry/opentelemetry-collector).
+2. Start MATLAB. 
+3. Ensure the installation directory of OpenTelemetry-matlab is on the MATLAB path.
+4. Run metrics_example.

examples/metrics/metrics_example.m

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+function metrics_example
+% This example creates 3 metric instruments including a counter, an 
+% updowncounter, and a histogram. It then enters a loop and updates the 
+% value of the instruments at each iteration.
+
+% Copyright 2023 The MathWorks, Inc.
+
+% initialize meter provider during first run
+runOnce(@initMetrics);
+
+% create meter and instruments
+m = opentelemetry.metrics.getMeter("metrics_example");
+c = createCounter(m, "counter");
+u = createUpDownCounter(m, "updowncounter");
+h = createHistogram(m, "histogram");
+iterations = 20;    % number of iterations
+
+for i = 1:iterations
+    c.add(randi(10));
+    u.add(randi([-10 10]));
+    h.record(50 + 15*randn);  % normal distribution with mean 50 and std 15
+    pause(5);
+end
+end
+
+
+function initMetrics
+% set up global MeterProvider
+exp = opentelemetry.exporters.otlp.defaultMetricExporter();
+reader = opentelemetry.sdk.metrics.PeriodicExportingMetricReader(exp, ...
+    "Interval", seconds(5), "Timeout", seconds(2.5));  % exports every 5 seconds
+% Use custom histogram bins
+v = opentelemetry.sdk.metrics.View(InstrumentType="histogram", HistogramBinEdges=0:10:100);
+mp = opentelemetry.sdk.metrics.MeterProvider(reader, View=v);
+setMeterProvider(mp);
+end
+
+% This helper ensures the input function is only run once
+function runOnce(fh)
+persistent hasrun
+if isempty(hasrun)
+    feval(fh);
+    hasrun = 1;
+end
+end