FastAPI middleware for request timing and metrics

<p>I want to measure API request duration and expose it as a Prometheus metric. I need middleware that times every request, records the route, method, and status code, and exposes a /metrics endpoint for Prometheus scraping.</p>

<p>Add Prometheus middleware with request duration histogram:</p> <div class="highlight"><pre><span></span><code><span class="c1"># pip install prometheus-client</span> <span class="kn">from</span><span class="w"> </span><span class="nn">prometheus_client</span><span class="w"> </span><span class="kn">import</span> <span class="n">Histogram</span><span class="p">,</span> <span class="n">Counter</span><span class="p">,</span> <span class="n">generate_latest</span><span class="p">,</span> <span class="n">CONTENT_TYPE_LATEST</span> <span class="kn">from</span><span class="w"> </span><span class="nn">fastapi</span><span class="w"> </span><span class="kn">import</span> <span class="n">FastAPI</span><span class="p">,</span> <span class="n">Request</span><span class="p">,</span> <span class="n">Response</span> <span class="kn">from</span><span class="w"> </span><span class="nn">fastapi.routing</span><span class="w"> </span><span class="kn">import</span> <span class="n">APIRoute</span> <span class="kn">import</span><span class="w"> </span><span class="nn">time</span> <span class="n">REQUEST_DURATION</span> <span class="o">=</span> <span class="n">Histogram</span><span class="p">(</span> <span class="s1">'http_request_duration_seconds'</span><span class="p">,</span> <span class="s1">'HTTP request duration'</span><span class="p">,</span> <span class="p">[</span><span class="s1">'method'</span><span class="p">,</span> <span class="s1">'route'</span><span class="p">,</span> <span class="s1">'status_code'</span><span class="p">],</span> <span class="n">buckets</span><span class="o">=</span><span class="p">[</span><span class="mf">0.01</span><span class="p">,</span> <span class="mf">0.05</span><span class="p">,</span> <span class="mf">0.1</span><span class="p">,</span> <span class="mf">0.5</span><span class="p">,</span> <span class="mf">1.0</span><span class="p">,</span> <span class="mf">5.0</span><span class="p">],</span> <span class="p">)</span> <span class="nd">@app</span><span class="o">.</span><span class="n">middleware</span><span class="p">(</span><span class="s1">'http'</span><span class="p">)</span> <span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">metrics_middleware</span><span class="p">(</span><span class="n">request</span><span class="p">:</span> <span class="n">Request</span><span class="p">,</span> <span class="n">call_next</span><span class="p">):</span> <span class="n">start</span> <span class="o">=</span> <span class="n">time</span><span class="o">.</span><span class="n">perf_counter</span><span class="p">()</span> <span class="n">response</span> <span class="o">=</span> <span class="k">await</span> <span class="n">call_next</span><span class="p">(</span><span class="n">request</span><span class="p">)</span> <span class="n">duration</span> <span class="o">=</span> <span class="n">time</span><span class="o">.</span><span class="n">perf_counter</span><span class="p">()</span> <span class="o">-</span> <span class="n">start</span> <span class="c1"># Get route template (e.g., /traces/{trace_id} not /traces/abc-123)</span> <span class="n">route</span> <span class="o">=</span> <span class="n">request</span><span class="o">.</span><span class="n">scope</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="s1">'route'</span><span class="p">)</span> <span class="n">path</span> <span class="o">=</span> <span class="n">route</span><span class="o">.</span><span class="n">path</span> <span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">route</span><span class="p">,</span> <span class="n">APIRoute</span><span class="p">)</span> <span class="k">else</span> <span class="n">request</span><span class="o">.</span><span class="n">url</span><span class="o">.</span><span class="n">path</span> <span class="n">REQUEST_DURATION</span><span class="o">.</span><span class="n">labels</span><span class="p">(</span> <span class="n">method</span><span class="o">=</span><span class="n">request</span><span class="o">.</span><span class="n">method</span><span class="p">,</span> <span class="n">route</span><span class="o">=</span><span class="n">path</span><span class="p">,</span> <span class="n">status_code</span><span class="o">=</span><span class="nb">str</span><span class="p">(</span><span class="n">response</span><span class="o">.</span><span class="n">status_code</span><span class="p">),</span> <span class="p">)</span><span class="o">.</span><span class="n">observe</span><span class="p">(</span><span class="n">duration</span><span class="p">)</span> <span class="k">return</span> <span class="n">response</span> <span class="nd">@app</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="s1">'/metrics'</span><span class="p">,</span> <span class="n">include_in_schema</span><span class="o">=</span><span class="kc">False</span><span class="p">)</span> <span class="k">async</span> <span class="k">def</span><span class="w"> </span><span class="nf">metrics</span><span class="p">():</span> <span class="k">return</span> <span class="n">Response</span><span class="p">(</span><span class="n">generate_latest</span><span class="p">(),</span> <span class="n">media_type</span><span class="o">=</span><span class="n">CONTENT_TYPE_LATEST</span><span class="p">)</span> </code></pre></div> <p>Key points: - Use route template (<code>/traces/{id}</code>) not actual URL to avoid high cardinality - <code>time.perf_counter()</code> is more accurate than <code>time.time()</code> for short durations - Avoid creating new metrics inside request handlers — define at module level - Exclude <code>/metrics</code> and <code>/health</code> from your own metrics to avoid noise</p>