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.

cmd/registry/main.go

@@ -16,6 +16,7 @@ import (
 	"github.com/modelcontextprotocol/registry/internal/database"
 	"github.com/modelcontextprotocol/registry/internal/model"
 	"github.com/modelcontextprotocol/registry/internal/service"
+	"github.com/modelcontextprotocol/registry/internal/telemetry"
 )
 
 func main() {
@@ -91,8 +92,20 @@ func main() {
 		}
 	}
 
+	shutdownTelemetry, metrics, err := telemetry.InitMetrics(cfg.Version)
+	if err != nil {
+		log.Printf("Failed to initialize metrics: %v", err)
+		return
+	}
+
+	defer func() {
+		if err := shutdownTelemetry(context.Background()); err != nil {
+			log.Printf("Failed to shutdown telemetry: %v", err)
+		}
+	}()
+
 	// Initialize HTTP server
-	server := api.NewServer(cfg, registryService)
+	server := api.NewServer(cfg, registryService, metrics)
 
 	// Start server in a goroutine so it doesn't block signal handling
 	go func() {