Add OpenTelemetry metrics instrumentation (#290)

## Motivation and Context
- It is important to have basic observability in place so that
availability issues can be figured out easily

## How Has This Been Tested?
- Registration of metrics is tested
- Prometheus endpoint `/metrics` is included in test cases to test the
metrics exposed
- Test locally to understand any regression.

## Breaking Changes
- No

## Types of changes
<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update

## Checklist
<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->
- [x] I have read the [MCP
Documentation](https://modelcontextprotocol.io)
- [x] My code follows the repository's style guidelines
- [x] New and existing tests pass locally
- [x] I have added appropriate error handling
- [ ] I have added or updated documentation as needed

## Additional context
- Telemetry implementation is currently done by keeping metrics in mind
as of now, other types of telemetry can be added on top of it later.
- Instrumentation takes care of these problems already:
- Cardinality control by limiting latency buckets and not including
labels that grows with throughput.
  - Latency buckets are added as per SLA guidelines
  - Each metric prefixed with `mcp` to easily identify.
- Pulling mechanism is used since the idea is to scrape metrics via an
agent like vmagent, prometheus agent or otel-collector.
- Metrics follow standard approach of defining counters, histogram and
gauges as per opentelemetry guidelines.

internal/telemetry/metrics_test.go

@@ -0,0 +1,84 @@
+package telemetry_test
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"go.opentelemetry.io/otel/exporters/prometheus"
+	"go.opentelemetry.io/otel/metric"
+	"go.opentelemetry.io/otel/metric/noop"
+	sdkmetric "go.opentelemetry.io/otel/sdk/metric"
+	"go.opentelemetry.io/otel/sdk/resource"
+
+	"github.com/modelcontextprotocol/registry/internal/telemetry"
+)
+
+func TestNewMetrics(t *testing.T) {
+	tests := []struct {
+		name           string
+		meter          metric.Meter
+		expectedMetric string
+		expectedError  error
+	}{
+		{
+			name:           "Valid Meter",
+			meter:          noop.NewMeterProvider().Meter("test"),
+			expectedMetric: "http_requests_total",
+			expectedError:  nil,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			metrics, err := telemetry.NewMetrics(tt.meter)
+
+			// Ensure the metric is registered correctly
+			assert.NoError(t, err)
+			assert.NotNil(t, metrics)
+			assert.NotNil(t, metrics.Requests)
+		})
+	}
+}
+
+func TestNewPrometheusMeterProvider(t *testing.T) {
+	tests := []struct {
+		name           string
+		wantErr        bool
+		mockPrometheus func() (*prometheus.Exporter, error)
+		expectedError  error
+	}{
+		{
+			name:    "Successful creation of meter provider",
+			wantErr: false,
+			mockPrometheus: func() (*prometheus.Exporter, error) {
+				return &prometheus.Exporter{
+					Reader: sdkmetric.NewManualReader(),
+				}, nil
+			},
+		},
+		{
+			name:          "Error on resource creation",
+			wantErr:       true,
+			expectedError: errors.New("error creating prometheus resource"),
+			mockPrometheus: func() (*prometheus.Exporter, error) {
+				return nil, errors.New("error creating prometheus resource")
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			res := resource.NewSchemaless() // Use an empty resource for testing.
+			exp, _ := tt.mockPrometheus()
+			mp, err := telemetry.NewPrometheusMeterProvider(res, exp)
+
+			if tt.wantErr {
+				assert.NotNil(t, err)
+			} else {
+				assert.NoError(t, err)
+				assert.NotNil(t, mp)
+			}
+		})
+	}
+}