在Angular 10中，报告进度事件总数未定义，但Content-Length标头存在的问题可能是由于服务器未正确设置Content-Length标头引起的。为了解决这个问题，可以尝试以下方法：

1. 使用HttpInterceptor拦截器：

import { Injectable } from '@angular/core';
import { HttpEvent, HttpInterceptor, HttpHandler, HttpRequest, HttpResponse } from '@angular/common/http';
import { Observable } from 'rxjs';
import { tap } from 'rxjs/operators';

@Injectable()
export class ContentLengthInterceptor implements HttpInterceptor {
  intercept(req: HttpRequest, next: HttpHandler): Observable> {
    return next.handle(req).pipe(
      tap(event => {
        if (event instanceof HttpResponse) {
          const contentLength = event.headers.get('Content-Length');
          if (contentLength === null && event.body) {
            event = event.clone({ headers: event.headers.set('Content-Length', event.body.length.toString()) });
          }
        }
      })
    );
  }
}

然后，在app.module.ts文件中将该拦截器提供给应用程序：

import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';
import { HttpClientModule, HTTP_INTERCEPTORS } from '@angular/common/http';

import { AppComponent } from './app.component';
import { ContentLengthInterceptor } from './content-length.interceptor';

@NgModule({
  declarations: [
    AppComponent
  ],
  imports: [
    BrowserModule,
    HttpClientModule
  ],
  providers: [
    { provide: HTTP_INTERCEPTORS, useClass: ContentLengthInterceptor, multi: true }
  ],
  bootstrap: [AppComponent]
})
export class AppModule { }

2. 使用rxjs的tap操作符：

import { Component } from '@angular/core';
import { HttpClient, HttpResponse } from '@angular/common/http';
import { tap } from 'rxjs/operators';

@Component({
  selector: 'app-root',
  template: `
    Get Data
  `
})
export class AppComponent {
  constructor(private http: HttpClient) { }

  getData() {
    this.http.get('https://api.example.com/data', { observe: 'response' })
      .pipe(
        tap((event: HttpResponse) => {
          const contentLength = event.headers.get('Content-Length');
          if (contentLength === null && event.body) {
            event = event.clone({ headers: event.headers.set('Content-Length', event.body.length.toString()) });
          }
        })
      )
      .subscribe(response => {
        console.log(response);
      });
  }
}

这些解决方法将在请求的响应中检查Content-Length标头是否存在，如果不存在但请求主体存在，则将Content-Length标头设置为请求主体的长度。这样就可以解决报告进度事件总数未定义的问题。